RIQ and SROIQ are Harder than SHOIQ


 Hubert Wilson
 2 years ago
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1 RIQ and SROIQ are Harder than SHOIQ Yegeny Kazako Oxford Uniersity Computing Laboratory, Wolfson Building, Parks Road, Oxford, O1 3QD, UK Abstract We identify the computational complexity of (finite model) reasoning in the sublanguages of the description logic SROIQ the logic currently proposed as the basis for the next ersion of the web ontology language OWL. We proe that the classical reasoning problems are N2ExpTimecomplete for SROIQ and 2ExpTimehard for its sublanguage RIQ. RIQ and SROIQ are thus exponentially harder than SHIQ and SHOIQ. The growth in complexity is due to complex role inclusion axioms of the form R 1 R n R, which are known to cause an exponential blowup in the tableaubased procedures for RIQ and SROIQ. Our complexity results, thus, also proe that this blowup is unaoidable. We also demonstrate that the hardness results hold already for linear role inclusion axioms of the form R 1 R 2 R 1 and R 1 R 2 R 2. Introduction In this paper we study the complexity of reasoning in sublanguages of SROIQ the logic chosen as the basis for the next ersion of the web ontology language OWL OWL 2. 1 SROIQ has been introduced in (Horrocks, Kutz, and Sattler 2006) as an extension of SRIQ (Horrocks, Kutz, and Sattler 2005), which itself is an extension of RIQ (Horrocks and Sattler 2003; 2004). For eery of these logics a corresponding tableaubased procedure has been proided. In contrast to sublanguages of SHOIQ whose computational properties are currently well understood (Tobies 2001), the complexity of languages between RIQ and SROIQ has been rather unexplored: it is known that RIQ and SRIQ are ExpTimehard as extensions of SHIQ, and SROIQ is NExpTimehard as an extension of SHOIQ. The difficulty in extending the existing complexity proofs to RIQ and SROIQ are caused by complex role inclusion axioms of the form R 1 R n R. The unrestricted usage of such axioms easily leads to undecidability of modal and description logics (Baldoni, Giordano, and Martelli 1998; Demri 2001; Horrocks and Sattler 2004), with the notable exception of EL ++ (Baader 2003; Baader, Brandt, and Lutz Unless 2ExpTime = NExpTime, in which case just SROIQ is harder than SHOIQ because NExpTime N2ExpTime Copyright c 2008, Association for the Adancement of Artificial Intelligence (www.aaai.org). All rights resered. 1 A.k.a. OWL 1.1:http://www.webont.org/owl/ ). Therefore, in order to ensure decidability, special syntactic restrictions hae been imposed on complex role inclusion axioms in RIQ. In a nutshell, the restrictions ensure that the axioms R 1 R n R when iewed as production rules of contextfree grammars R R 1...R n, induce regular languages a property that has been used before to characterize a decidable class of multimodal logic called regular grammar logics (del Cerro and Panttonen 1988; Demri 2001; Demri and de Nielle 2005). The tableau procedure for RIQ works with complex role inclusion axioms ia the corresponding regular automata for these languages. Unfortunately, the size of the automata can be exponential in the number of axioms, which results in an exponential blowup in the worstcase behaiour of the procedure for RIQ in comparison to the procedure for SHIQ. It has been an open problem whether this blowup can be aoided (Horrocks and Sattler 2003). In this paper we demonstrate that RIQ and SROIQ are indeed exponentially harder than respectiely SHIQ and SHOIQ, which implies that the blowup in the tableau procedures could not be aoided. This paper is an extended ersion of (Kazako 2008) containing new results on linear role inclusion axioms. Preliminaries We assume that the reader is familiar with the DL SHOIQ (Horrocks and Sattler 2007). A SHOIQ signature is a tuple Σ = (C Σ, R Σ, I Σ ) consisting of the sets of atomic concepts C Σ, atomic roles R Σ and indiiduals I Σ. A role is either some r R Σ or an inerse role r. For each r R Σ, we set In(r) = r and In(r ) = r. A SHOIQ RBox is a finite set R of role inclusion axioms (RIA) R 1 R, transitiity axioms Tra(R) and functionality axioms Fun(R) where R 1 and R are roles. Let R be the smallest reflexie transitie relation on roles such that R 1 R R implies R 1 R R and In(R 1 ) R In(R). A role S is called simple w.r.t. R if there is no role R such that R R S and either Tra(R) R or Tra(In(R)) R. Gien an RBox R, the set of SHOIQ concepts is the smallest set containing,, A, {a}, C, C D, C D, R.C, R.C, ns.c, and ns.c, where A is an atomic concept, a an indiidual, C and D concepts, R a role, S a simple role w.r.t. R, and n a nonnegatie integer. A SHOIQ TBox is a finite set T of general concept inclusion axioms (GCIs) C D where C and D are concepts. We write C D as an abbreiation for
2 C D and D C. A SHOIQ ABox is a finite set consisting of concept assertions C(a) and role assertions R(a, b) where a and b are indiiduals from I Σ. A SHOIQ ontology is a triple O = (R, T, A), where R a SHOIQ RBox, T is a SHOIQ TBox, and A is a SHOIQ ABox. SHIQ is a sublanguage of SHOIQ that does not use nominals {a}. A SHOIQ interpretation is a pair I = ( I, I) where I is a nonempty set called the domain of I, and I is the interpretation function, which assigns to eery A C Σ a subset A I I, to eery r R Σ a relation r I I I, and to eery a I Σ, an element a I I. The interpretation I is finite iff I is finite. I is extended to complex role, complex concepts, axioms, and assertions in the usual way (Horrocks and Sattler 2007). I is a model of a SHOIQ ontology O, if eery axiom and assertion in O is satisfied in I. O is (finitely) satisfiable if there exists a (finite) model I for O. A concept C is (finitely) satisfiable w.r.t. O if C I for some (finite) model I of O. The problem of (concept) satisfiability is ExpTimecomplete for SHIQ, and NExpTimecomplete for SHOIQ (see, e.g., Tobies 2000; 2001). 2 RIQ (Horrocks and Sattler 2004) extends SHIQ with complex RIAs in RBoxes of the form R 1 R n R which are interpreted as R I 1 R I n R I, where is the usual composition of binary relations. A regular order on roles is an irreflexie transitie binary relation on roles such that R 1 R 2 iff In(R 1 ) R 2. A RIA R 1 R n R is said to be regular, if either: (i) n = 2 and R 1 = R 2 = R, or (ii) n = 1 and R 1 = In(R), or (iii) R i R for 1 i n, or (i) R 1 = R and R i R for 1 < i n, or () R n = R and R i R for 1 i < n. 3 A RIQ RBox R is regular if there exists a regular order on roles such that each RIA from R is regular. A RIQ ontology can contain only a regular RBox R. The notion of simple role is extended in RIQ as follows. Let R be the smallest relation such that R 1 R n R R if either n = 1 and R 1 = R, or there exist 1 i j n and R such that R 1 R i 1 R R j+1 R n R R and R i... R j R R or In(R j )... In(R i ) R R. A role S is simple w.r.t. R if there are no roles R 1,...,R n with n 2 such that R 1 R n R S. SRIQ (Horrocks, Kutz, and Sattler 2005) further extends RIQ with: (1) the uniersal role U, which is interpreted as the total relation: U I = I I, and cannot occur in complex RIAs, (2) negatie role assertions R(a, b), (3) the concept constructor S.Self interpreted as {x I x, x S I } where S is a simple role, (4) the new role axioms Sym(R), Ref(R), Asy(S), Irr(S), Disj(S 1, S 2 ) where S (i) are simple roles, which restrict R I to be symmetric or reflexie, S I to be asymmetric or irreflexie, or S I 1 and S I 2 to be disjoint. SROIQ (Horrocks, Kutz, and Sattler 2006) extends SRIQ with nominals like in SHOIQ. 2 For further information and references on complexities of DLs seehttp://www.cs.man.ac.uk/ ezolin/dl/ 3 The original definition of RIQ (Horrocks and Sattler 2003), admits only RIAs R 1 R n R with n 2; in this paper we assume the definition for RIQ from (Horrocks and Sattler 2004) The Exponential Blowup for Regular RIAs In this section we discuss the main reason why the tableau procedures for RIQ, SRIQ, and SROIQ in (Horrocks and Sattler 2004; Horrocks, Kutz, and Sattler 2005; 2006) incur an exponential blowup. Gien an RBox R containing complex RIAs and a role R, let L R (R) be the language defined by: L R (R) := {R 1 R 2... R n R 1 R n R R} (1) It has been shown in (Horrocks and Sattler 2004) that for eery regular RBox R and eery role R the language L R (R) is regular. The tableau procedures for RIQ and SROIQ, utilize nondeterministic finite automata (NFA) corresponding to L R (R) to ensure that only finitely many states are produced by the tableau expansion rules. Unfortunately, the NFA for L R (R) can be exponentially large in the size of R, which results in exponential blowup in the number of states produced in the worst case by the procedure for RIQ and SROIQ compared to the procedures for SHIQ and SHOIQ. It was conjectured in (Horrocks, Kutz, and Sattler 2006) that without further restrictions on RIAs such blowup is unaoidable. In Example 1, we demonstrate that minimal automata for regular RBoxes can be exponentially large. Example 1. Let R be an RBox consisting of the RIA (2). r r (2) The RIA (2) is not regular regardless of the ordering. Indeed, (2) does not satisfy conditions (i) (ii) of  regularity since n = 3, and it does not satisfy conditions (iii) (i) since = R 2 R =. It is easy to see that L R (s) = {r i r i i 0}, where r i denotes the word consisting of i letters r. Thus the language L R () is nonregular, which can be shown, e.g., by using the pumping lemma for regular languages (see, e.g., Sipser 2005). As an example of a regular RBox, consider the RIAs (3) oer the atomic roles 0,..., n. i i i+1 0 i < n (3) It is easy to see that these axioms satisfy condition (iii) of regularity for eery ordering such that i j, for eery 0 i < j n. By induction on i, it is easy to show that L R ( i ) consist of finitely many words, and hence, are all regular. It is also easy to show that j 0 L R( i ) iff j = 2 i. Let Q( i ) be an NFA for L R ( i ) and q 0,..., q j a run in Q( i ) accepting j 0 for j = 2i. Then all states in this run are different, since otherwise there is a cycle, which would mean that Q( i ) accepts infinitely many words. Hence Q( i ) has at least j + 1 = 2 i + 1 states. Although Example 1 does not demonstrate the usage of the conditions (i), (ii), (i) and () for regularity of RIAs, as will be shown in the next section, axioms that satisfy just the condition (iii) already make reasoning in RIQ and SROIQ hard. The Lower Complexity Bounds In this section, we present two hardness results for fragments of SROIQ. First, we proe that reasoning in R a fragment of RIQ that does not use counting and inerse roles is 2ExpTimehard. The proof is by reduction from the word
3 c I (x) = 00 r 01 r 10 r 11 B 2 B 1 B 2 B 1 B 2 B 1 B 2 B 1 Figure 1: Encoding exponentially long chains problem for an exponentialspace alternating Turing machine. Second, we demonstrate that reasoning in ROIF the extension of R with nominals, inerse roles and functional roles is N2ExpTimehard. The proof of this result is by reduction from the doublyexponential Domino tiling problem. The main idea of our reductions is to enforce doubleexponentially long chains using axioms in the DL R. Singleexponentially long chains can be enforced using a wellknown integer counting technique (Tobies 2000). A counter c I (x) is an integer between 0 and 2 n 1 assigned to an element x of the interpretation I using n atomic concepts B 1,..., B n as follows: the k th bit of c I (x) is equal to 1 if and only if x B k I. It is easy to see that axioms (4) (8) induce an exponentially long rchain by initializing the counter and incrementing it oer the role r (see Figure 1). Z B 1 B n (4) E B 1 B n (5) E r. (6) (B 1 r. B 1 ) ( B 1 r.b 1 ) (7) B k 1 r. B k 1 (B k r. B k ) ( B k r.b k ) (8) 1 < k n Axiom (4) is responsible for initializing the counter to zero using the atomic concept Z. Axiom (5) can be used to detect whether the counter has reached the final alue 2 n 1, by checking whether E holds. Thus, using axiom (6), we can express that eery element whose counter has not reached the final alue has an rsuccessor. Axioms (7) and (8) express how the counter is incremented oer r: axiom (7) expresses that the lowest bit of the counter is always flipped; axioms (8) express that any other bit of the counter is flipped if and only if the lower bit is changed from 1 to 0. Lemma 2. Let O be an ontology containing axioms (4) (8). Then for eery model I = ( I, I) of O and x Z I there exist x i I with 0 i < 2 n such that x = x 0, x i 1, x i r I when 1 i < 2 n, and c I (x i ) = i. Proof. We construct the required x i I with 0 i < 2 n by induction on i and simultaneously show that c I (x i ) = i. For the base case i = 0 we take x 0 := x. Since I is a model of (4) and x Z I, we hae c I (x 0 ) = 0. For the induction step, assume that we hae constructed x i with 1 i < 2 n 1 and c I (x i ) = i. We construct x i+1 and proe that c I (x i+1 ) = i + 1. Consider i+1 = {x x i, x r I }. Since I is a model of (6) and c I (x i ) 2 n 1, we hae that x i / E I, and therefore there exists some x i+1 i+1. Now we demonstrate that c I (x) = i+1 for eery x i+1, and in particular for x = x i+1. By induction on k with 1 k n, we proe that for eery x i+1, the k th bits of c I (x i ) and of c I (x) differ O 2 n r r r n n n n r r r n n n n 2 2n Figure 2: Encoding a doubleexponentially long chain if and only if the (k 1) th bit of c I (x i ) is 1 and of c I (x) is 0. Note that, in particular, the induction hypothesis implies that the alues for the k th bits of c I (x) are the same for all x i+1. The base case k = 1 of induction holds since I is a model of (7), and therefore, for eery x i+1 the lowest bits of c I (x i ) and of c I (x) differ. The induction step hols because I is a model of (8) which implies that the (k 1) th bit of c I (x i ) is 1 and of c I (x) is 0 for all x i+1 if and only if for eery x i+1, the k th bit of c I (x i ) and of c I (x) differ. Now we use similar ideas to enforce doubleexponentially long chains in the model. This time, howeer, we cannot use just atomic concepts to encode the bits of the counter since there are exponentially many bits. Therefore, we assign a counter not to elements but to exponentially long rchains induced by axioms (4) (8) using one atomic concept : the i th bit of the counter corresponds to the alue of at the i th element of the chain. In Figure 2 we hae depicted a doublyexponential chain formed for the sake of presentation as a zigzag that we are going to induce using R axioms. The chain consists of 2 2n rchains, each haing exactly 2 n elements, that are joined together using a role the last element of eery rchain, except for the final chain, is  connected to the first element of the next rchain. The tricky part is to ensure that the counters corresponding to rchains are properly incremented. This is achieed by using the regular role inclusion axioms from (3), which allow us to propagate information using a role n across chains of exactly 2 n roles. The structure in Figure 2 is enforced using axioms (9) (18) in addition to axioms (3) (8). O Z Z (9) Z r.z (10) Z E E r.e (11) (E ) r. E (12)
4 E r.(y Y f ) Y f (26) r. (Y Y f ) Y f (27) Y f (Y h n. Y ) ( Y h n.y ) (28) Y f (Y h n.y ) ( Y h n. Y ) (29) The grid structure in Figure 3 is now enforced by adding axioms (30) (33). ( h n.) ( h n.) (30) (Y n.y ) ( Y n. Y ) (31) E E E h Y {a} (32) Fun(r ) Fun( ) Fun(h ) (33) Axioms (30) and (31) express that the alues of the ertical / horizontal counters are copied across h n / respectiely n. Axiom (32) expresses that the last element of the r chain with the final coordinates is unique. Together with axiom (33) expressing that the roles r,, and h are inerse functional, this ensures that no two different rchains hae the same coordinates. Note that the roles r,, and h are simple because they do not occur at the right hand side of RIAs (3), (14), (24), and (25). The following analog of Lemma 2 claims that the models of our axioms that satisfy O correspond to the grid in Figure 3. Lemma 4. For eery model I = ( I, I) of eery ontology O containing axioms (3) (33), and eery x O I there exist x i,j,k I with 0 i < 2 n, 0 j, k < 2 2n such that (i) x = x 0,0,0, (ii) x i 1,j,k, x i,j,k r I when i 1, (iii) x 2n 1,j 1,k, x 0,j,k I when j 1, and (i) x 2n 1,j,k 1, x 0,j,k h I when k 1. Proof. By induction on j + k with 0 j, k < 2 2n, we construct nonempty sets of elements i,j,k I for 0 i < 2 n such that (a) x 0,0,0, (b) x i 1,j,k i 1,j,k x i,j,k i,j,k and x i,j,k i,j,k x i 1,j,k i 1,j,k such that x i 1,j,k, x i,j,k r I when i 1, (c) x 2n 1,j 1,k 2n 1,j 1,k x 0,j,k 0,j,k such that x 2n 1,j 1,k, x 2n 1,j,k I when j 1, and (d) x 2n 1,j,k 1 2n 1,j,k 1 x 0,j,k 0,j,k such that x 2n 1,j,k 1, x 0,j,k h I when k 1. We also proe by induction that for eery x i,j,k, we hae (e) c I (x) = i, (f) x I iff j[2 n i] 2 = 1, and (g) x Y I iff k[2 n i] 2 = 1. After that, we demonstrate that eery set i,j,k contains exactly 1 element which we define by x i,j,k. For the base case j = k = 0, we construct sets i,0,0 as follows. Since I is a model of (9), we hae x O I Z I. By Lemma 2, there exist elements x i I with 0 i < 2 n such that c I (x i ) = i, x = x 0, and x i 1, x i r I when i 1. We define i,0,0 := {x i } for 0 i n. It is easy to see that conditions (a), (b), and (e) are satisfied for the constructed sets. Since I is a model of (9), (10), (19), and (20), we hae x i / I and x i / Y I for eery i with 0 i < 2 n, and therefore the conditions (f) and (g) are satisfied for i,0,0. For the induction step j + k > 0, we construct i,j,k proided we hae constructed all i,j,k with j +k < j+k and 0 i < 2 n. We first initialize i,j,k to the empty set, and then add new elements as described below. If j 1, by the induction hypothesis (b), for eery element x 2n 1,j 1,k 2n 1,j 1,k there exist elements x i,j 1,k i,j 1,k with 0 i < 2 n 1 such that x i 1,j 1,k, x i,j 1,k r I when i 1. By the induction hypothesis (e) we hae c I (x i,j 1,k ) = i. Since j 1 < 2 2n 1, by Lemma 3, there exist elements x i,j,k with 0 i < 2 n 1 such that x 2n 1,j 1,k, x 0,j,k I, x i 1,j,k, x i,j,k r I when i > 1, c I (x i,j,k ) = i, and x i,j,k I iff j[2 n i] 2 = 1. Since I is a model of (3), (14), and (31), it is also easy to show that x i,j,k Y I iff x i,j 1,k Y I iff k[2 n i] 2 = 1. We add eery constructed element x i,j,k with 0 i < 2 n to the corresponding set i,j,k. We hae demonstrated that the properties (b), and (e) (g) hold for each of these elements. Similarly, if k 1, by the induction hypothesis (b), for eery element x 2n 1,j,k 1 2n 1,j,k 1, there exist elements x i,j,k 1 i,j,k 1 with 0 i < 2 n 1 such that x i 1,j,k 1, x i,j,k 1 r I when i 1. By applying the analog of Lemma 3 where is replaced with h, we construct elements x i,j,k with 0 i < 2 n such that x 2n 1,j,k 1, x 0,j,k h I, and the properties (d) (g) are satisfied. We add eery constructed element x i,j,k to the corresponding set i,j,k. Note that since either j 1 and i,j 1,k is nonempty, or k 1 and i,j,k 1 is nonempty, the constructed set i,j,k is nonempty as well. Now after all sets i,j,k are constructed, it is easy to see that the conditions (c) and (d) are satisfied as well. It remains thus to proe that eery set i,j,k contains exactly one element. Fist, consider the set i,j,k for i = 2 n 1 and j = k = 2 2n 1. By condition (b), for eery element x i,j,k i,j,k there exist elements x i,j,k i,j,k with 0 i < 2 n 1 such that x i 1,j,k, x i,j,k ri when i 1 and c I (x i,j,k ) = i. Since I is a model of (11) and (21), it can be shown using condition (f) and (g) that x i,j,k (E E E h Y ) I. Now, since I is a model of (32), we hae x i,j,k = ai, and therefore i,j,k contains exactly one element. Since I is a model of (33), using conditions (b), (c), and (d), it is easy to show that each set i,j,k with 0 i < 2 n and 0 j, k < 2 2n contains at most one element. Our complexity result for ROIF is obtained by a reduction from the bounded domino tiling problem. A domino system is a triple D = (T, V, H), where T = {1,...,p} is a finite set of tiles and H, V T T are horizontal and ertical matching relations. A tiling of m m for a domino system D with initial condition c 0 = t 0 1,..., t0 n, t0 i T, 1 i n, is a mapping t : {1,...,m} {1,..., m} T such that t(i 1, j), t(i, j) V, 1 < i m, 1 j m, t(i, j 1), t(i, j) H, 1 < i m, 1 j m, and t(1, j) = t 0 j, 1 j n. It is well known (Börger, Grädel, and Gureich 1997) that there exists a domino system D 0 that is N2ExpTimecomplete for the following decision problem: gien an initial condition c 0 of the size n, check if D 0 admits the tiling of 2 2n 2 2n for c 0. Axioms
5 (34) (41) in addition to axioms (3) (33) proide a reduction from this problem to the problem of concept satisfiability in ROIF. D 1 D p (34) D i D j 1 i < j p (35) D i r.d i 1 i p (36) D i.d j i, j V (37) D i h.d j i, j H (38) O I 1 (39) I k D t 0 k r.i k 1 k n (40) I k h.i k+1 1 k < n (41) The atomic concepts D 1,..., D p correspond to the tiles of the domino system D 0. Axioms (34) and (35) express that eery element in the model is assigned with a unique tile D i. Axiom (36) expresses that the elements of the same rchain are assigned with the same tile. Axioms (37) and (38) express the ertical and horizontal matching properties. Finally, axioms (39) (41) expresses that the initial condition holds for the first row. It is easy to see that this reduction is polynomial in n since D 0 is fixed. Theorem 5. Let c 0 be an initial condition of size n for the domino system D 0 and O an ontology consisting of the axioms (3) (41). Then D 0 admits the tiling of 2 2n 2 2n for c 0 if and only if O is (finitely) satisfiable in O. Proof. ( ) Let t : 2 2n 2 2n T be a tiling for the domino system D 0 = (T, V, H) with the initial condition c 0. We use t to build a finite model I = ( I, I) for O that satisfies O. We define I := {x i,j,k 0 i < 2 n, 0 j, k < 2 2n }. The interpretation of the roles r,, and h is defined by r I = { x i 1,j,k, x i,j,k i 1}, I = { x 2n 1,j 1,k, x 0,j,k j 1}, h I = { x 2n 1,j,k 1, x 0,j,k k 1}. The roles l and h l for 0 l n are interpreted as the smallest relations that satisfy axioms (3), (14), (24), and (25). It is easy to see that I n = { x i,j 1,k, x i,j,k j 1} and h I n = { x i,j,k 1, x i,j,k k 1}. We interpret concepts B k with 1 k n that determine the bits of the counter c I (x) in such a way that c I (x i,j,k ) = i. Thus Z I = {x 0,j,k }, Z I = {x 2n 1,j,k}. We define I = {x i,j,k j[2 n i] 2 = 1}, and Y I = {x i,j,k k[2 n i] 2 = 1}. Finally, we define D I l = {x i,j,k t(j + 1, k + 1) = l}. Other concepts such as Z, Z h, E, E h, f, Y f, I k are interpreted in a clear way. It is straightforward to check that I satisfies all axioms in O. In particular, I satisfies (37) and (38), since t satisfies the matching conditions V and H of D 0, and the roles and h connect only the corresponding consequent rchains. ( ) Let I be a model of O that satisfies O. By Lemma 4, there exist x i,j,k I with 0 i < 2 n, 0 j, k < 2 2n that satisfy the conditions (i) (i) of Lemma 4. Let us define a function t : 2 2n 2 2n {1,...,p} by setting t(j, k) = l if and only if x 0,j 1,k 1 D I l. This function is defined correctly because I satisfies axioms (34) and (35). We demonstrate that t is a tiling for D 0 = (T, H, V ) with the initial condition c 0. In order to proe that t satisfies the initial condition c 0, we show by induction on k that x 0,0,k 1 I I k for all k with 1 k n. Since I is a model of (40), it follows then that x 0,0,k 1 D I t 0 k, and hence t(1, k) = t 0 k by definition of t(j, k). The base case of induction k = 1 holds since I is a model of (39) and by condition (i) of Lemma 4 we hae x 0,0,0 = x O I I I 1. For the induction step, assume that x 0,0,k 1 I I k for some k with 1 k < 2 n, and let us show that x 0,0,k I I k+1. By condition (ii) of Lemma 4, x i 1,0,k 1, x i,0,k 1 r I for all i with 1 i < 2 n. Therefore, since I is a model of (40), we hae x i,0,k 1 I I k for all i with 0 i < 2 n and, in particular, x 2n 1,0,k 1 I I k. Since I is a model of (41), and x 2n 1,0,k 1, x 0,0,k h I by condition (i) of Lemma 4, we hae x 0,0,k I I k+1 what was required to show. Finally we proe that t satisfies the matching conditions H and V of D 0. If t(j 1, k) = l 1 and t(j, k) = l 2 for some j > 1, 1 j, k < 2 2n, then by definition of t(j, k), we hae x 0,j 2,k 1 D I l1 and x 0,j 1,k 1 D I l2. Furthermore, since I is a model of (36) and by condition (ii) of Lemma 4, x i 1,j 2,k 1, x i,j 2,k 1 r I when i 1, we hae x i,j 2,k 1 D I l1 for eery i with 0 i < 2 n, and in particular x 2n 1,j 2,k 1 D I l1. By condition (iii) of Lemma 4, we hae x 2n 1,j 2,k 1, x 0,j 1,k 1 I. Since x 2 n 1,j 2,k 1 D I l1, x 0,j 1,k 1 D I l2, and I is a model of (37), we hae t(j 1, k), t(j, k) = l 1, l 2 V. Therefore t satisfies the ertical matching condition. Analogously using condition (i) of Lemma 4 and axiom (38) it is easy to show that t satisfies the horizontal matching condition. Corollary 6. The problem of (finite) concept satisfiability in the DL ROIF is N2ExpTimehard (and so are all the standard reasoning problems). R is 2ExpTimehard In this section, we proe that (finite model) reasoning in the DL R is 2ExpTimehard. The proof is by reduction from the word problem of an exponentialspace alternating Turing machine. The main idea of our reduction is to use the zigzaglike structures in Figure 2 to simulate a computation of an alternating Turing machine. An alternating Turning machine (ATM) is a tuple M = (Γ, Σ, Q, q 0, δ 1, δ 2 ) where Γ is a finite working alphabet containing a blank symbol ; Σ Γ \ { } is the input alphabet; Q = Q Q {q a } {q r } is a finite set of states partitioned into existential states Q, uniersal states Q, an accepting state q a and a rejecting state q r ; q 0 Q is the starting state, and δ 1, δ 2 : (Q Q ) Γ Q Γ {L, R} are transition functions. A configuration of M is a word c = w 1 qw 2 where w 1, w 2 Γ and q Q. An initial configuration is c 0 = q 0 w 0 where w 0 Σ. The size c of a configuration c is the number of symbols in c. The successor configurations δ 1 (c) and δ 2 (c) of a configuration c = w 1 qw 2 with q q a, q r oer the transition functions δ 1 and δ 2 are defined like for deterministic Turing machines
6 (see, e.g., Sipser 2005). The sets C a (M) of accepting configurations and C r (M) of rejecting configurations of M are the smallest sets such that (i) c = w 1 qw 2 C a (M) if either q = q a, or q Q and δ 1 (c), δ 2 (c) C a (M), or q Q and δ 1 (c) C a (M) or δ 2 (c) C a (M), and (ii) c = w 1 qw 2 C r (M) if either q = q r, or q Q and δ 1 (c), δ 2 (c) C r (M), or q Q and δ 1 (c) C r (M) or δ 2 (c) C r (M). The set of configurations reachable from an initial configuration c 0 in M is the smallest set M(c 0 ) such that c 0 M(c 0 ) and δ 1 (c), δ 2 (c) M(c 0 ) for eery c M(c 0 ). A word problem for an ATM M is to decide gien an initial configuration c 0 whether c 0 C a (M). M is g(n) space bounded if for eery initial configuration c 0 we hae: (i) c 0 C a (M) C r (M), and (ii) c g( c 0 ) for eery c M(c 0 ). It follows from a classical complexity result AExpSpace = 2ExpTime (Chandra, Kozen, and Stockmeyer 1981) that there exists a 2 n space bounded ATM M 0 for which the word problem is 2ExpTimecomplete. In order to reduce the word problem of M 0 to reasoning problems in R, we introduce an auxiliary notion of a computation of an ATM that is more conenient to deal with when determining accepting computations. Let us denote by {0, 1} the set of all finite words oer the letters 0 and 1, by ǫ the empty word, and for eery b {0, 1}, by b 0 and b 1 a word obtain by appending 0 and 1 to b. A subset B {0, 1} is prefixclosed if b 1 B or b 0 B implies b B. A computation of an ATM M from c 0 is a pair P = (B, π), where B {0, 1} is a prefixclosed set, and π : B M(c 0 ) a mapping from words to configurations reachable from c 0, such that (i) π(ǫ) = c 0, and for eery b B with π(b) = c = w 1 qw 2 we hae (ii) q q r, (iii) q Q implies {b 0, b 1} B, π(b 0) = δ 1 (c), and π(b 1) = δ 2 (c), and (i) q Q implies b 0 B and π(b 0) = δ 1 (c), or b 1 B and π(b 1) = δ 2 (c). A computation is finite if B is finite. It is easy to see that for any g(n) space bounded ATM M, we hae c 0 C a (M) iff there exists a finite computation of M from c 0. Let c 0 be an initial configuration of M 0 and n = c 0 (w.l.o.g., n 3). In order to decide whether c 0 C a (M 0 ), we introduce axioms expressing the existence of a computation of M 0 from c 0. The axioms induce a treelike structure depicted in Figure 4 that stores configurations of M 0 on 2 n  long rchains. The rchain starting from the root element stores the initial configuration c 0 ; eery configuration, depending on its state has up to two successor configurations stored on rchains reachable by roles and h an rchain corresponding to c is connected to rchains corresponding to δ 1 (c) and δ 2 (c) ia the roles and h in a similar way as in Figure 2. For encoding configurations, we introduce an atomic concept A s for eery s from the set of states Q and the working alphabet Γ of M 0. We also introduce two concepts S and that are used to mark configurations haing existential and uniersal states. The underlying treelike structure of the computation is induced by axioms (42) (50). O Z (42) O A c 0 1 r.(a c 0 2 ( r.a c 0 n r.z ) ) (43) Z A r.z (44) S r h S O r S S h r q a h Figure 4: Encoding a computation of an ATM A q S q Q (45) A q q Q (46) S r.s r. (47) A qr (48) E S.Z h.z (49) E.Z h.z (50) Axioms (42) (44) initialize the configuration c 0 on the r chain starting from the origin O of the structure. Axioms (45) (46) determine the uniersal and existential types of configurations from their states. Axioms (47) then propagate these types until the end of the rchain. Axiom (48) forbids rejecting configuration in the computation. Finally, axioms (49) and (50) express the existence of successor configurations depending on the types of the configuration. In order to express that the successor configurations are obtained by the transition functions δ 1 and δ 2, we are going to use the roles n and h n that connect the corresponding elements of successor rchains thanks to axioms (3), (14), (24), and (25). It is a wellknown property of the transition functions of Turing machines that the symbols c 1 i and c2 i at the position i of δ 1 (c) and δ 2 (c) are uniquely determined by the symbols c i 1, c i, c i+1, and c i+2 of c at the positions i 1, i, i + 1, and i We assume that this correspondence is gien by the (partial) functions λ 1 and λ 2 such that λ 1 (c i 1, c i, c i+1, c i+2 ) = c 1 i and λ 2(c i 1, c i, c i+1, c i+2 ) = c 2 i. To encode these functions, for eery quadruple of symbols s 1, s 2, s 3, s 4 Q Γ, we introduce a concept S s1s 2s 3s 4 that expresses the neighborhood of an element in an r chain it expresses that the current element is assigned with s 2, its rpredecessor with s 1 and its next two rsuccessors with s 3 and s 4 (s 1, s 3, and s 4 are if there are no such elements). Axioms (51) (56) below express the required properties of the transition functions. Z A s2 r.(a s3 r.a s4 ) S s2s 3s 4 (51) 4 If any of the indexes i 1, i + 1, or i + 2 are out of range for the configuration c, we assume that the corresponding symbols c i 1, c i+1, or c i+2 are the blank symbol
7 A s1 r.(a s2 r.(a s3 r.a s4 )) r.s s1s 2s 3s 4 (52) A s1 r.(a s2 r.(a s3 E)) r.s s1s 2s 3 (53) A s1 r.(a s2 E) r.s s1s 2 (54) S s1s 2s 3s 4 n.a λ1(s 1,s 2,s 3,s 4) (55) S s1s 2s 3s 4 h n.a λ2(s 1,s 2,s 3,s 4) (56) Axioms (51) (54) initialize concepts S s1s 2s 3s 4. Axioms (55) (56) express that the corresponding symbols in the successor rchains are computed using the functions λ 1 and λ2. Theorem 7. Let O be an ontology consisting of axioms (3) (8), (14), (24), (25), and (42) (56). Then c 0 C a (M 0 ) if and only if O is (finitely) satisfiable in O. Proof. ( ) Assume that c 0 C a (M 0 ). Since M 0 is 2 n space bounded, there exists a finite computation P = (B, π) of M 0 from c 0 such that π(b) 2 n for eery b B. We will use this computation in order to guide the construction of a finite model I = ( I, I) for O that satisfies O. We define I := {x b,i b B, 0 i < 2 n }. The interpretation of roles r,, and h is defined by r I = { x b,i 1, x b,i i 1}, I = { x b,2 n 1, x b 0,0 b 0 B}, h I = { x b,2n 1, x b 1,0 b 1 B}. The roles k and h k for 0 k n are interpreted as the smallest relations that satisfy axioms (3), (14), (24), and (25). It is easy to see that I n = { x b,i, x b 0,i b 0 B} and h I n = { x b,i, x b 1,i b 1 B}. We interpret concepts B k with 1 k n that determine the bits of the counter c I (x) in such a way that c I (x b,i ) = i. Thus Z I = {x b,0 b B}, E I = {x b,2n 1 b B}. For eery s Q Γ we define A I s = {x b,i π(b) i = s}, where π(b) i denotes the i th symbol in the configuration π(b). We set S I = {x b,i j : π(b) j Q } and S I = {x b,i j : π(b) j Q }. Other concepts such as Z and S s1s 2s 3s 4 are interpreted in a clear way. It is straightforward to check that I satisfies all axioms in O. In particular, I satisfies (55) and (56), since n and h n connect only the corresponding elements of rchains. ( ) Assume that I is a model of O. We build a computation P = (B, π) of M 0 from c 0 witnessed by I. The elements b B and the alues π(b) are built inductiely on b for b B, together with elements x b,i I with 0 i < 2 n. We demonstrate by induction that c I (x b,i ) = i, (x b,i 1, x b,i ) r I when i 1, and the property ( ): π(b) i = s implies x b,i A I s for 0 i < 2 n, where as before, π(b) i denotes the i th symbol of the configuration π(b) (we assume that π(b) i = if i > π(b) ). For the base case b = ǫ, we define x ǫ,0 := x O and π(ǫ) := c 0. Since I is a model of (42), we hae x ǫ,0 Z I. Since I is a model of (4) (8), by Lemma 2, there exist elements x ǫ,i I with 1 i < 2 n such that x ǫ,i 1, x ǫ,i r I and c I (x ǫ,i ) = i. The property ( ) for b = ǫ holds since I is a model of (43) and (44). Now assume that we hae constructed some b B, all elements x b,i I with 1 i < 2 n, and the alue of π(b). Let π(b) j = q Q be the state of the configuration π(b) occurring at the position j. By the property ( ), we hae x b,j A I q. If q Q, then since I is a model of (45) and (47), we hae x b,2n 1 S I. Since I is a model of (49), there exists either x b 0,0 Z I such that x b,2n 1, x b 0,0 I, or x b 1,0 Z I such that x b,2n 1, x b 1,0 h I. In either case we add the respectie elements b 0 or b 1 to B. If q Q then similarly, since I is a model of (46), (47), and (50), there exist x b 0,0, x b 1,0 Z I such that x b,2n 1, x b 0,0 I and x b,2 n 1, x b 1,0 h I. In this case, we add both elements b 0 and b 1 to B. Note that it is not possible that q = q r since I is a model of (48). If we add element b 0 to B then we define π(b 0) := δ 1 (π(b)). Since x b 0,0 Z I and I is a model of (4) (8), by Lemma 2, there exist elements x b 0,i I with 1 i < 2 n such that x b 0,i 1, x b 0,i r I and c I (x b 0,i ) = i. Similarly, if we add b 1 to B then we define π(b 1) := δ 2 (π(b)) and find elements x b 1,i I. It remains thus to show the property ( ) for the new elements in B. If b 0 B then since x b,2n 1, x b 0,0 I and I is a model of (3) and (14), we hae x b,i, x b 0,i I n for eery i with 0 i < 2 n. Since I is a model of (51) (55) and function λ 1 correspond to the transition function δ 1, we obtain ( ) for b 0. Similarly, if b 1 B then since x b,2n 1, x b 1,0 h I and I is a model of (24), (25), (51) (54), and (56) we hae ( ) for b 1. Corollary 8. The problem of (finite) concept satisfiability in the DL R is 2ExpTimehard (and so are all the standard reasoning problems). Hardness Results for Linear RIAs In this section we sharpen our hardness results for the linear RIAs of the form R 1 R 2 R 2 (rightlinear) or R 2 R 1 R 2 (leftlinear) that were considered in the original definition of RIQ (Horrocks and Sattler 2003). We say that an RBox R is linear regular if R is regular and eery complex RIA in R is linear. Since linear regular RBoxes can already proide many desirable features for modeling of biomedical ontologies (e.g., propagation of properties oer properties) but still cause an exponential blowup in the size of regular automata, the question about the exact computational complexity of RIQ and SROIQ with linear RIAs becomes apparent. In this section we demonstrate how our hardness proofs for RIQ and SROIQ can be adapted for linear RIAs. Linear regular RBoxes are strictly less expressie than the general ones: for eery linear RBox R, wheneer R 1 R n R R holds for some n 2, then either R 1 R 1 R n R R or R 1 R n R n R R holds as well. In particular, the key property ( 0 ) i R n iff i = 2 n used in our construction cannot be expressed using linear RIAs. Therefore we use a slightly more complicated construction in Figure 5 to connect the corresponding elements of rchains. Instead of connecting rchains using a single role, we now connect them using an exponential chain. Moreoer, the rchains and the chains are now composed of seeral roles r 0,..., r n 1 and 0,..., n respectiely. Intuitiely, a role r k ( k ) connects elements when the (k + 1) th bit of the counter is changed from 1 to 0. The chains are created using axioms (57) (64) that replace
8 r r 3 r r 1 4 l 4 r r 2 r 0 r 1 r 0 r 2 r 0 r 1 r 0 r 3 r 0 r 1 r 0 r 2 r 0 r 1 r 0 r r 2 r3 r 2 l l 3 r r 1 1 l r 2 r l 2 3 l 1 l 0 r r0 r 2 r r l 2 r 4 4 r 0 r 1 r 0 r 2 r 0 r 1 r 0 r 3 r 0 r 1 r 0 r 2 r 0 r 1 r 0 Figure 5: Connecting the corresponding elements of rchains using linear RIAs r l 0 r l 2 axioms (6), (13), and (14). O V (57) V B k l<k B l r k 1. V 1 k n (58) V B k l<k B l k 1.V 1 k n (59) V E (E ) n.(z V ) (60) V E n.(z V ) (61) r k r k 0 k < n (62) (B 1. B 1 ) ( B 1.B 1 ) (63) B k 1. B k 1 (B k. B k ) ( B k.b k ) (64) 1 < k n We use a new concept V to distinguish elements in chains from the elements of rchains. Axiom (57) expresses that the origin of our zigzaglike structure belongs to an rchain. Axioms (58) and (59) construct the successor elements of the chains, whereby (58) replaces (6). Axioms (60) and (61) create successor chains and replace axiom (13). Axiom (62) initializes the roles r and which are used to increment the counters in (7) (8) and (63) (64). To connect the corresponding elements of the chains, we use RIAs (65) (70). r k r l k r k r r k k l k k r k (65) l k k l k k r k r k k < k (66) r l k r k r l k r k r r k r r k k < k (67) r l k r k r l k l k r r k rr k k < k (68) l k rr k l k r l k r k r k (69) r k l k (70) We hae introduced new atomic roles r l k, rr k, l k and r k with 0 k < n and 0 k n. These roles are implied by r k and k using axioms (65), and are propagated using leftand rightlinear RIAs (66) and (67) oer r k and k with smaller indexes. Thus, eery element of an rchain (chain) has at most one r l k (l k )successor and at most one rr k (r k ) predecessor for eery k (see Figure 5). RIAs (68) and (69) are used to connect the corresponding elements of r and chains. Intuitiely, this is done by first going ia r l k (rr k ) in the rchain and then going ia the corresponding r k (l k ) in the chain and connecting the resulting elements. These axioms together with (70) ensure that connects only those elements of successor chains that correspond. It is easy to see that RIAs (65) (70) are regular for any ordering such that 0 r 0 n r n r l n rr n l n r n r l 0 rr 0 l 0 r 0. Now to complete the construction, we replace in the remaining axioms eery concept of the form n.c with V..(V C), which says that C holds at eery successor of an element when both elements belong to rchains (and hence, correspond). Our modified construction proes the following theorem: Theorem 9. The standard reasoning problems in R and ROIF are respectiely 2ExpTimehard and N2ExpTimehard een for linear regular RBoxes. SROIQ is in N2ExpTime In this section we proe the matching upper complexity bound for reasoning in SROIQ using an exponentialtime translation into the two ariable fragment with counting C 2. Let O be a SROIQ ontology for which we need to test satisfiability. By Theorem 9 from (Horrocks, Kutz, and Sattler 2006), w.l.o.g., we can assume that O does not contain concept and role assertions, the uniersal role, and axioms of the form Irr(S), Tra(R), and Sym(R). We can also assume that O contains no Ref(R) or Asy(S): we replace eery Ref(R) with s R and s.self for a fresh (simple) atomic role s, and replace eery Asy(S) with Disj(S, In(S)). Next, we conert O into the simplified form which contains only axioms of the form 1 10 in Table 1, where A (i) and B (j) are atomic concepts, r (i) atomic roles, s (i) simple atomic roles, and nonsimple atomic roles. The transformation can be done in polynomial time using the standard structural transformation which iteratiely introduces definitions for compound subconcept and subroles (see, e.g., Kazako and Motik 2008). For example, the axiom A r.{a} is replaced with the axioms A 1s.A a, s r, A a {a}, and A a r.a where s is a fresh (simple) atomic role introduced for r, and A a a fresh atomic concepts introduced for {a}. After the transformation, we eliminate RIAs of the form 10 using a technique from (Demri and de Nielle 2005).
9 SROIQ axiom firstorder translation 1. A r.b x.(a(x) y.[r(x, y) B(y)]) 2. A n s.b x.(a(x) n y.[s(x, y) B(y)]) 3. A n s.b x.(a(x) n y.[s(x, y) B(y)]) 4. A s.self x.(a(x) s(x, x)) 5. A a {a} 6. Ai B j =1 y.a a (y) x.( A i (x) B j (x)) 7. Disj(s 1, s 2 ) xy.(s 1 (x, y) s 2 (x, y) ) 8. s 1 s 2 xy.(s 1 (x, y) s 2 (x, y)) 9. s 1 s 2 xy.(s 1 (x, y) s 2 (y, x)) 10. r 1 r n, n 1 Table 1: Translation of simplified SROIQ axioms into C 2 Such axioms can cause unsatisfiability of O only through axioms of the form 1, since other axioms do not contain nonsimple roles. In particular, for eery r 1 r n R, the axiom A.B implies axiom (71) below: A r r n.b (71) The main idea of the transformation is to use the NFA for L R () to express all properties of the form (71) using additional noncompositional axioms. Let B R () be an NFA for L R () with the set of states Q, starting state q 0 Q, accepting states F Q, and transition relation δ Q R Σ Q. Gien B R (), we replace eery axiom A.B of the form 1 with axioms (72) (74) where A q is a fresh atomic concept for eery q Q. A A q q = q 0 (72) A q 1 s.a q 2 q 1, s, q 2 δ (73) A q B q F (74) Lemma 10. Let O be an ontology consisting of axioms of the form 1 10 from Table 1, and O obtained from O by replacing eery axiom A.B of the form 1 with axioms (72) (74) and remoing all axioms of the form 10. Then (1) eery model of O can be expanded to a model of O by interpreting A q, and (2) eery model of O can be expanded to a model of O by interpreting the nonsimple roles in O. Proof. (1) Let I be a model of O and I an expansion of I satisfying all axioms of the form (72) and (73) in O that interprets A q as smallest possible sets I can be constructed as a fixed point of the transformation that expands the interpretation according to these axioms. We proe that I also satisfies all axioms of the form (74), and, therefore, is a model of O. Assume, to the contrary, I does not satisfy some axiom A q B of the form (74), that is, there exists x I such that x (A q) I but x / B I. Since I is obtained by a fixed point construction, there exists a sequence of elements x 0,..., x n = x in I such that x 0 A I, x i 1, x i s I i for 1 i n, and the word s 1...s n is accepted by B R (). Since B R () is an automaton for L R (), we hae s 1... s n L R (), and so s 1 s n R by definition of L R (), which implies that x 0, x n I. Since x 0 A I and I = A.B, we obtain x n B I which contradicts the assumption x / B I since x = x n and B I = B I. The proof by contradiction implies that I is a model of O. (2) Let I be a model of O and I an expansion of I that interprets all nonsimple roles as smallest possible relations that satisfy all axioms in O of the form 10 I can be constructed by a fixed point from I. We claim that I is a model of O. Indeed, I is a model of all axioms in O that do not contain nonsimple roles (in particular those of the form 2 9). It remains thus to demonstrate that I satisfies all axioms A.B of the form 1 where is a nonsimple role. Let x A I and x, y I ; we need to proe that y B I. By the construction of I, there exists a sequence of elements x = x 0,..., x n = y in I such that x i 1, x i s i I for some simple role s i, 1 i n, and s 1 s n R. The last implies that s 1...s n is accepted by some run of q 0, q 1,...,q n F of the automaton B R (). Since O contains the corresponding axioms of the form (72) (74) that are satisfied by I, it is easy to show by induction on i that x i (A r q i ) I, which implies that y = x n (A r q n ) I B I, which was required to proe. Theorem 11. The problem of (finite) satisfiability of SROIQ ontologies is solable in N2ExpTime (and so are all the standard reasoning problems). Proof. The input SROIQ ontology O can be translated in exponential time (due to the sizes of the automata) presering (finite) satisfiability into a simplified ontology containing only axioms of the form 1 9 in Table 1, which in turn can be translated into the two ariable fragment with counting quantifiers C 2 according to the second column of Table 1. Since (finite) satisfiability of C 2 is NExpTimecomplete (PrattHartmann 2005), our reduction proes that (finite) satisfiability of SROIQ is in N2ExpTime. Conclusions In this paper we hae identified the computational complexity of (finite model) reasoning in SROIQ to be N2ExpTimecomplete, and in RIQ and SRIQ to be 2ExpTimehard that is, exponentially harder then for SHOIQ and SHIQ respectiely. The complexity blowup is due to complex role inclusion axioms, and in particular due to their ability to chain a fixed exponential number of roles. The blowup occurs een when no other new constructors in SROIQ are used, such as (as)symmetric, (ir)reflexie, disjoint, uniersal roles, or self constructor, and for RIQ, een without number restrictions and inerse roles. Moreoer, we hae demonstrated that our hardness results hold already for complex role inclusion axioms of the form R 1 R 2 R 1 and R 1 R 2 R 2 that hae been originally introduced in RIQ. Seeral conclusions can be immediately drawn from our complexity results. First, they demonstrate that the exponential blowup in the existing tableaubased procedures for RIQ and SROIQ (Horrocks and Sattler 2004; Horrocks, Kutz, and Sattler 2006) are unaoidable. That is, there is essentially no better way of dealing with complex RIAs other
10 SH[O]IQ ABox TBox RBox NP 5 [N]ExpTime [N]ExpTime SR[O]IQ ABox TBox RBox NP 5 [N]ExpTime 6 [N]2ExpTime 6 Table 2: Parametrized complexity of SRIQ and SROIQ than representing them by (possibly exponential) automata. Second, the complexity results imply that SROIQ, and therefore OWL 2 proide for strictly richer expressiity than SHOIN and respectiely OWL DL up to now there were no formal eidences that the new constructors in SROIQ could not be polynomially expressed in SHOIN. Finally, it remains, as usual, to point out that the high worstcase complexity of the logic tells little about the typical behaiour of the reasoning procedures. As has been demonstrated, only RIAs are responsible for the exponential blowup. In particular, the complexity of SROIQ is N2ExpTime in the size of the ontology but only NExpTime in the size of the TBox (see Table 2). Since in existing ontologies the size of the RBox is typically much smaller than the size of the TBox, the impact on the complexity of the procedures might not be as dramatic as it sounds. In our proofs, the worstcase situations are achieed by a rather artificial usage of RIAs which is also unlikely to occur in real ontologies. In fact, as discussed in (Horrocks and Sattler 2004) the exponential blowup does not take place in many natural cases, e.g., when the length of eery sequence R 1 R 2 is bounded. The existing tableaubased procedures are already aware of these cases and therefore exhibit a pay as you go behaior. There are seeral interesting questions and problems that can be considered for future work. First, we did not obtain the upper complexity bound for RIQ and SRIQ. We think that a matching 2ExpTime decision procedure can be easily obtained by the elimination of complex RIAs as described in this paper followed by the modification of of the ExpTime automaton for SHIQ (Tobies 2001) to include other new constructors of SRIQ, such as r.self. Second, there is a gap between regular RBoxes and regular grammars that needs to be filled in. For example, the axioms R 1 R 1 R 2, R 2 R 2 R 2, R 1 R 2 R 1, and R 2 R 1 R 1 correspond to a regular grammar, but cannot be captured by a regular RBox. Finally, it would be interesting to identify other practically releant restrictions of regular RBoxes which do not result in the exponential blowup. For example, it is not clear from our results whether RIQ or SROIQ are still exponentially harder than SHIQ and SHOIQ when the complex RIAs are only leftlinear or only rightlinear. 5 The data complexity of SHOIQ is currently not known but is generally belieed to be NPcomplete; we conjecture that the data complexity of SRIQ and SROIQ is also NPcomplete 6 For SRIQ only hardness results are proed in this paper References Baader, F.; Brandt, S.; and Lutz, C Pushing the EL enelope. In Proc. IJCAI05, Baader, F Restricted rolealuemaps in a description logic with existential restrictions and terminological cycles. In Proceedings of the 2003 International Workshop on Description Logics (DL2003), CEURWS. Baldoni, M.; Giordano, L.; and Martelli, A A tableau for multimodal logics and some (un)decidability results. In TABLEAU, Börger, E.; Grädel, E.; and Gureich, Y The Classical Decision Problem. Perspecties of Mathematical Logic. Springer. Second printing (Uniersitext) Chandra, A. K.; Kozen, D. C.; and Stockmeyer, L. J Alternation. J. ACM 28(1): del Cerro, L. F., and Panttonen, M Grammar logics. Logique et Analyse : Demri, S., and de Nielle, H Deciding regular grammar logics with conerse through firstorder logic. Journal of Logic, Language and Information 14(3): Demri, S The complexity of regularity in grammar logics and related modal logics. J. Log. Comput. 11(6): Horrocks, I., and Sattler, U Decidability of SHIQ with complex role inclusion axioms. In IJCAI, Horrocks, I., and Sattler, U Decidability of SHIQ with complex role inclusion axioms. Artif. Intell. 160(12): Horrocks, I., and Sattler, U A tableau decision procedure for SHOIQ. J. Autom. Reasoning 39(3): Horrocks, I.; Kutz, O.; and Sattler, U The irresistible SRIQ. In Proc. of the First OWL Experiences and Directions Workshop. Horrocks, I.; Kutz, O.; and Sattler, U The een more irresistible SROIQ. In KR, Kazako, Y., and Motik, B A resolutionbased decision procedure for SHOIQ. Journal of Automated Reasoning 40(23): Kazako, Y SRIQ and SROIQ are harder than SHOIQ. In DL PrattHartmann, I Complexity of the twoariable fragment with counting quantifiers. Journal of Logic, Language and Information 14(3): Sipser, M Introduction to the Theory of Computation, Second Edition. Course Technology. Tobies, S The complexity of reasoning with cardinality restrictions and nominals in expressie description logics. J. Artif. Intell. Res. (JAIR) 12: Tobies, S Complexity Results and Practical Algorithms for Logics in Knowledge Representation. Ph.D. Dissertation, RWTH Aachen, Germany.
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