# 1 Definitions. Supplementary Material for: Digraphs. Concept graphs

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1 Supplementary Material for: van Rooij, I., Evans, P., Müller, M., Gedge, J. & Wareham, T. (2008). Identifying Sources of Intractability in Cognitive Models: An Illustration using Analogical Structure Mapping. In: Proceedings of the 30 th Annual Meeting of the Cognitive Science Society. 1 Definitions Digraphs A directed graph or digraph, for short, G is a pair (V, A) of a nonempty set of vertices V and a set of arcs A V 2 of ordered pairs (v, w) of two distinct vertices v, w V. Such a pair is informally understood as an arc pointing from v to w. A digraph is acyclic, or a DAG, if and only if it does not contain directed cycles. A subdag of DAG G is a DAG G = (V, A ) such that A A and V V. We call G induced (by V ) if and only if A = A (V V ). Let G = (V, A) be a DAG. A root of G is a vertex with no incoming arcs. A leaf of G is a vertex with no outgoing arcs. A vertex which is not a leaf is internal. A vertex v is a child of a vertex w if and only if (w, v) A. A vertex v is a descendent of a vertex w if and only if there is a directed path from w to v. The height of G is the length (number of arcs) of a longest directed path in G. The graph underlying G is the (undirected) graph (V, E) with E = {{v, w} (v, w) A}, i.e. the graph obtained when we forget the direction of the arcs in G. A component of G is a subdag induced by the vertices of a component of the graph underlying G. A directed tree is a DAG such that for any two vertices v, w V there is at most one directed path from v to w. A poly-tree is a DAG G such that the graph underlying G is a forest. Concept graphs A concept graph is a quadruple (G, λ A, λ B, λ P ) for a DAG G = (V, A) and functions λ A, λ B, λ P called labelings such that 1. λ A : A N, 2. λ B is injective and defined on the leafs of G, 3. λ P is defined on the internal vertices of G, 4. If v is an internal vertex, then λ A either enumerates the set of arcs leaving v or is constantly 0 on this set. In the first case v is ordered, in the second unordered. 5. for internal vertices u, v with λ P (u) = λ P (v) the following holds: (a) either both u and v are ordered or both u and v are unordered, (b) u and v have the same number of children in G, (c) {(v, λ A (v, v )) (v, v ) A} = {(u, λ A (u, u )) (u, u ) A}. Usually we denote the range of λ B by B and the range of λ P by P. Elements of B are called entities, those of P predicates or relations. A predicate is ordered if and only if at least one vertex (equivalently: all vertices) labeled with it is ordered. A concept graph is ordered (unordered) if and only if all its vertices are ordered (unordered). 1

2 Analogy morphisms Let G = (V, A) and G = (V, A ) be two DAGs. A subdag isomorphism from G to G is an isomorphism f of a subdag of G onto a subdag of G. We write G f = (V f, A f ) for the subdag on the domain of f and call it the subdag associated with f. Let G := (G, λ A, λ P, λ B ) and G = (G, λ A, λ P, λ B ) be two concept graphs. An analogy morphism of G and G is a subdag isomorphism from G to G satisfying the following three conditions: 1. for all v V f also all children of v in G are in V f. 2. λ P (f(v)) = λ P (v) for all v V f. 3. λ A ((f(v), f(w))) = λ A((v, w)) for all (v, w) A f. The value of analogy morphisms Let a concept graphs G = (G, λ A, λ P, λ B ) be given. Relative to a function pval : P N and two naturals lm, trd N we associate a valuation val mapping vertices of G to a value in N. This function is defined inductively over the height of the vertex in G: the value of a vertex v is match(v) + trd val(w) (w,v) A where match(v) is pval(λ P (v)) if v is an internal vertex and lm if v is a leaf. The value val(g ) of a subdag G = (V, A ) of G is v V val(v). The value of an analogy morphism f of two concept graphs is val(g f ). It is easy to see that the value of a given analogy morphism between two concept graphs can be computed in time polynomial in the size of the concept graphs. The problem The NP-optimization problem is Input: Two concept graphs G and G, a function pval : P N, where P are the predicates in G, and naturals lm, trd N. Solutions: All analogy morphisms between G and G Cost: The valuation val associated with pval, lm, trd. Goal: Maximization. The associated decision problem is Input: Two concept graphs G and G, a function pval : P N, where P are the predicates in G, naturals lm, trd N and a natural k N Question: Is there an analogy morphism between G and G of value at least k? Here it is understood that value refers to the valuation associated with pval, lm, trd. In our work we are concerned with the following slightly simplified version. Of course intractability of this simplified version immediately implies intractability of the non-simplified version. SMAD Input: Two concept graphs G and G. Problem: Is there an analogy morphism between G and G of value at least k? Here it shall be understood that value refers to the valuation associated with the function pval which is constantly one and the constants lm = trd = 1. We denote this valuation by val. 2

3 2 Complexity Results Classical complexity SUBGRAPH ISOMORPHISM Input: Two graphs G and H. Problem: Is H isomorphic to a subgraph of G? SUBFOREST ISOMORPHISM is the restriction of SUBGRAPH ISOMORPHISM to instances where G is a tree and H is a forest. Lemma 1 There is a polynomial time reduction from SUBGRAPH ISOMORPHISM to SMAD. Proof: Let G = (V, E) be a graph. We define the concept graph C(G) = (C(G), λ A, λ P, λ B) as follows. The DAG C(G) has vertices V E and arcs A := {(e, v) v e E}. λ A is constantly 0, λ P is constantly p (for some predicate p) on E and λ B is the identity on V. Let (G, H) be an instance of SUBGRAPH ISOMORPHISM. Then if f is an isomorphism from H onto a subgraph of G, then f is an analogy morphism between C(H) and C(G), where f is defined as follows: it maps all entities (vertices of H) as f does and additionally maps an edge {h, h } of H (which is also a vertex in C(H)) to {f(h), f(h )} (a vertex in C(G)). The value of f is the value of C(G). Conversely, if f is an analogy morphism between C(H) and C(G) with value val(c(h)), then its domain is the set of all vertices of C(H). Hence its restriction to the vertices of H is an isomorphism to a subgraph of G - why? To see this let {h, h } be an edge of H. This is a vertex in C(H). Since f preserves predicates, this vertex is mapped to a vertex in C(G) which is an edge {g, g } of G. By defnition of analogy morphisms, then {f(h), f(h )} equals {g, g }, and so is an edge of G. It follows that (G, H) (C(H), C(G), val(c(h))) defines a polynomial time reduction as claimed. It is easy to see that SMAD NP. Because SUBGRAPH ISOMORPHISM is famously NPcomplete, it follows Corollary 2 SMAD is NP-complete. The reduction given in the previous Lemma is robust enough to survive under various restrictions. For example, observe that if a graph G is a forest, then C(G) is a poly-tree. It is well-known that SUBFOREST ISOMORPHISM is NP-complete. It follows that Corollary 3 SMAD restricted to instances where the given concept graphs are poly-trees is NPcomplete. 3

4 Parameterized complexity In the paper we make several tractability and intractability claims numbered from 1 to 7. We prove them subsequently. All proofs of intractability use some common assumption from parameterized complexity. The assumption that W[1] FPT is strong enough for all our purposes. Claim 1 SMAD is fp-intractable for parameter set {h, a, f, s}. Proof: It is enough to show that SMAD is NP-hard even when instances are restricted to those where all parameters are required to be bounded by a constant. Then it follows that the parameterized problem is complete for the huge class paranp [1, Theorem 2.14] and is thus fixedparameter tactable if and only if P = NP. This in turn follows by a reduction due to Veale et al. [2] from the NP-complete problem 3-DIMENSIONAL MATCHING which produces concept graphs such that h = 1, a = 2, f = 1 and s = 0. Remark 4 As a matter of fact, the reduction in [2] produces ordered concept graphs. Claim 2 SMAD is fp-intractable for parameter set {n/h}. Here the parameter is to be understood to be max{n 1 /h 1, n 2 /h 2 }. Proof: For a concept graph G = (G, λ A, λ P, λ B ) define G as follows: say G has n vertices. We add to G a directed path with n vertices and an arc from the leaf of this path to a leaf of G. We label each new vertex with an own new predicate. All new arcs get label 1. Then G has 2n vertices and height n. Thus the parameter of this instance is at most 2. An instance (G, G, k) of SMAD is equivalent to ( G, G, k) (provided the new predicate labels chosen in the construction of G and G are different) because no analogy morphism between G and G can involve some of the new vertices. But the instance ( G, G, k) has parameter at most 2, so paranp-hardness follows as in the previous proof. Remark 5 The construction above preserves the property of being ordered, i.e. if G is ordered, then so is G. Claim 3 SMAD is fp-tractable for parameter set {n 1}. Proof: As we have explained in the paper, this is trivial. Claim 4 SMAD is fp-intractable for parameter set {n 2, r, h, a, p}. Proof: It is well-known that the parameterized problem p-clique Input: A graph G and a natural k N. Parameter: k. Problem: Does G contain a clique with k elements? is W[1]-hard. It thus suffices to give a parameterized reduction from this problem. Let C k be a clique with k vertices. G has a k clique if and only if (G, C k ) is a yes instance of SUBGRAPH ISOMORPHISM, hence by Lemma 1, if and only if (C k, G, val(c k )) is a yes instance of SMAD. The parameters of the instance produced are n 2 = k, a = 2, p = r = h = 1, all in O(k). 4

5 Claim 5 SMAD is fp-tractable for parameter set {o}. Proof: Let (G, G, l) be an instance of SMAD. Let k denote the maximum number of leafs in one of the given concept graphs. Let F be the set of bijections between a subset of leafs of G and a subset of leafs of G. Let g F. We stepwise extend this morphism. For each vertices v of level one in G there is at most one vertex v of level one in G such that extending g by mapping v to v is an analogy morphism. We make all possible such extensions. Then we proceed in the same way with the vertices in level two and so on. This way we generate in polynomial time an analogy morphism with the best possible value among those whose restriction to the leafs equal g. We compute this value for each g F and accept if we find a value l. Doing this amounts to F times a polynomial time computation. Because F can be effectively bounded in k, this is fpt time. Remark 6 Note that the proof proves Claim 7 in the paper, which also implies the weaker claim, Claim 5 in the paper. Claim 6 Claim 1 to 5 hold true when SMAD is restricted to instances with ordered concept graphs only. Proof: By Remarks 4 and 5 we are left to verify this for Claim 4. There in the proof we constructed an instance with unordered concept graphs C(C k ) and C(G). Fix an arbitrary linear order < on the vertices of G. We transform C(G) to an ordered concept graph C(G) by labeling an arc ({v, v }, v) in C(G) (for an edge {v, v } of G) with 1 if v < v and with 2 otherwise. Clearly any analogy morphism between any ordered version of C(C k ) and C(G) is also an analogy morphism between C(C k ) and C(G) of the same value. Conversely if there is an analogy morphism between C(C k ) and C(G) then this is also an analogy morphism between C(G) and some ordered version of C(C k ). Thus by the proof of Claim 4, we conclude that G has a k clique if and only if there is an ordered version C(C k ) of C(C k ) such that (C(C k ), C(G), val(c(c k ))) is a yes instance of SMAD. For the sake of contradiction assume that there is an fpt algorithm A solving SMAD with parameter set {n 2, r, h, a, p}. We get a contradiction by deriving that p-clique would then also be fixed-parameter tractable. By the latter condition of the above equivalence we get an fpt algorithm solving p-clique by simulating A on input (C(C k ), C(G), val(c(c k ))) for all possible ordered versions C(C k ) of C(C k ). As the number of such ordered versions can be effectively bounded in k, this amounts to an fpt running time. References [1] J. Flum and M. Grohe, Parameterized complexity theory, Springer, [2] T. Veale, D. O Donoghue, and M. T. Keane, Computability as a limiting cognitive constraint:complexity resources in metaphor comprehension about which cognitive linguistics should be aware, in E. M. Higara, C. Sinha, S. Wilcox (eds.), Cultural, psychological and typoogical issues in cognitive linguistics, John Benjamins, Amsterdam,

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