# Complexity problems over algebraic structures

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1 Complexity problems over algebraic structures Theses of PhD dissertation Created by: Horváth Gábor Mathematical Doctoral School Theor. Mathematical Doctoral Program Dir. of School: Dr. Laczkovich Miklós Dir. of Program: Dr. Szenthe János Supervisor: Dr. Szabó Csaba, DSc, associate professor Eötvös Loránd University Faculty of Natural Sciences 2010

2 Nowadays, computers play larger and larger role in everyday life and in scientic research. This is especially true in mathematics and algebra, where one often wants to perform calculations or computations with a machine. To determine whether or not two expressions are identically equal or whether an equation has a solution are important questions in the areas of universal algebra or in the theory of automata and formal languages. The equivalence problem over a nite algebra A asks whether or not two expressions p and q attain the same value for every substitution from A, i.e. whether or not p q is an identity over A. The equation solvability problem over A asks, whether or not the equation p = q has any solution over A. The input expressions can be terms, i.e. expressions that are built up from variables and the basic operations of A, or polynomials, i.e. expressions that are built up from variables, constants from A and the basic operations of A. Thus we distinguish four problems altogether: the term equivalence problem (shortly equivalence), the polynomial equivalence problem, the term equation solvability problem, and the polynomial equation solvability problem (shortly equation solvability). Each of these problems is decidable for a nite algebra A by checking all possible substitutions. The interesting question is therefore whether these problems can be solved fast, and what computational complexity they have. We investigate the complexity of these questions for nite rings and groups. Denition (Denition 2.1). The equivalence problem (for a nite algebra A) consists of two input term expressions s and t over A and asks whether or not s and t are equivalent over A, i.e. whether or not s t is an identity over A. The polynomial equivalence problem (for a nite algebra A) consists of two input polynomial expressions p and q over A and asks whether or not p and q are equivalent over A, i.e. whether or not p q is an identity over A. The equation solvability problem (for a nite algebra A) consists of two input polynomial expressions p and q over A and asks if there exists a substitution for which the functions p A and q A attain the same value, i.e. whether or not p = q has a solution over A. We do not examine the term equation solvability problem, because the 2

3 equation s = t always admits a trivial solution whenever s and t are terms over a ring or over a group. We consider the complexities of the equivalence, of the polynomial equivalence and of the equation solvability problems in the length of the input terms or polynomials. Denition (Denition 2.2). Let A be a nite algebra. We dene the length p of a term or polynomial expression p inductively. The length of a variable x or a constant c is 1: x = c = 1. For an m-ary basic operation f the length of the polynomial f (p 1,..., p m ), whenever it is dened, is the sum of p i, i.e. f (p 1,..., p m ) = m i=1 p i. The length of f (x 1,..., x m ) is m. Besides being interesting problems on their own, the equivalence and equation solvability problems have deep applications in other mathematical areas. An important universal algebraic question is to decide whether an algebra A is a member of a variety V. Here, the variety V can be given either by the identities that hold in V or by an algebra B that generate V. In the latter case an identity holds in V if and only if it holds in B. Thus the membership problem can be decided by simple determining for each identity whether or not it holds in A. The equation solvability problem is used in automata theory. Recognizing a language can be reduced to equation solvability over the syntactic monoid of the language. Early investigations into the equivalence problem for various nite algebraic structures were carried out by computer scientists, in particular at Syracuse University where the terminology the term equivalence problem was introduced. They considered nite commutative rings and nite lattices. Their motivation came from industry: the synchronization of various chemical experiments led to the equivalence problem over nite commutative rings. In the early 1990's it was shown by Hunt and Stearns (see [10]) that the equivalence problem over a nite commutative ring either has polynomial time complexity or is conp-complete. Later Burris and Lawrence proved in [6] that the same holds for rings in general. In their proof Burris and Lawrence reduced the SAT problem to the equivalence of polynomials written as products of sums. Nevertheless, these polynomials can be exponential long if are written in the usual ring-theoretic 3

4 form, i.e. as sums of monomials. This observation motivated Ross Willard in dening the sigma equivalence and sigma equation solvability problem, where the input polynomials are written as sums of monomials. Willard and Lawrence [12] conjectured that the sigma equivalence problem over a nite ring has polynomial time complexity whenever the factor by the Jacobson radical is commutative or conp-complete otherwise. Szabó and Vértesi proved the conp-complete part of the statement. In [14, 15, 16] they prove that the sigma equivalence problem is conp-complete over a nite ring if the factor by the Jacobson radical is not commuative. In Chapter 3 of the thesis we verify the conjecture for commutative rings. Theorem (Theorem 3.1). Let R be a nite commutative ring. equivalence problem over R has polynomial time complexity. Then the As of now, there are no published results about the sigma equation solvability problem. Nevertheless, using the method of Szabó and Vértesi it is easy to prove that the sigma equation solvability is NP-complete over a nite ring if the factor by the Jacobson radical is not commutative. In Chapter 3 we determine the complexity of the sigma equation solvability problem for certain nite rings. Theorem (Theorem 3.2). Let the nite ring R be the direct sum of nite elds, nilpotent rings and rings isomoprhic to Z 4. Then the sigma equivalence problem over R has polynomial time complexity. The equivalence and equation solvability over nite groups proved to be a more challenging topic than that for nite rings. The equivalence problem over a nite nilpotent group has polynomial time complexity by Burris and Lawrence [7]. Goldmann and Russell [8, 9] proved that the equations solvability problem over a nite nilpotent ring has polynomial time complexity. They reduced the equation solvability problem to recognizing languages by deterministic, non-uniform, nite automatons obtained from a nilpotent group. Such automatons have been investigated earlier by Péladeau and Thérien in [13]. Nevertheless, many of the properties required are not proved in the french paper [13], but in [17]. Hence the reader can easily get lost in trying to uncover the main idea behind the proof of Goldmann and Russel. In 4

5 Chapter 5 we give a direct proof, which is slightly clearer than the one behind the abovementioned three papers, it does not cite publications and the equivalence follows from it as well. Theorem (Theorem 5.2). Let G be a nilpotent group. The (polynomial) equivalence and equation solvability problems over G have polynomial time complexities. After investigating nilpotent groups, it is natural to consider nite groups being a semidirect product of two Abelian groups. Burris and Lawrence in [7] posed the problem of determining the complexity of the equivalence over every solvable, not nilpotent group. Ondrej Klíma investigated the equivalence and equation solvability problem over nite semigroups. In [11] he determined the complexity of these questions over every semigroup containing at most 6 elements, except for the group S 3. Goldmann and Russell explicitly ask in [8] to decide the complexity of solving an equation over S 3. In Chapter 6 we determine these complexities for some solvable, not nilpotent groups. We answer the questions of Klíma, Goldmann and Russel, and we take the rst step in answering the question of solvable, not nilpotent groups. Theorem (Corollary 6.3). The (polynomial) equivalence problem over any of the following groups has polynomial time complexity. 1. G = A B, where A and B are Abelian, 2. G = Z n B, where the (polynomial) equivalence over B has polynomial time complexity. 3. G = Z n1 (Z n2 (Z nk (A B))), where each n i is a positive integer and A, B are commutative groups. Theorem (Corollary 6.5). The equation solvability over any of the following groups has polynomial time complexity. 1. G = Z p B, where B is a nite commutative group and p is a prime; 2. G = Z 4 B, where B is a nite commutative group; 5

6 3. G = Z 2 2 B, where B is a nite commutative group, 2 B ; 4. G = Z 2 p Z 2, where p is an odd prime. Some of these results are published in [5] in English, in [1] in Hungarian. The proof we communicate in Chapter 6 reduces the equivalence and equation solvability problems over A B to the equivalence and equation solvability problems over the End A-module A. The equivalence and equation solvability problems have not yet been introduced. In Chapter 4 we dene modulepolynomials and module-monomials. Then we dene the sigma equivalence and sigma equation solvability problems over nite modules. Even though the results of Chapter 4 are interesting on their own, we mostly use them to prove other theorems. Thus we consider the results of Chapter 4 as useful techniques. The following theorems play an important role in proving the results of Chapter 6. Theorem (Theorem 4.1). Let R be a nite, commutative, unital ring and let M be a faithful R-module. Let S be the subgroup of R. If f is a modulepolynomial written as a sum of module-monomials, then it is decidable in polynomial time whether or not (R, M) = f 0 for substitutions from S. Theorem (Theorem 4.2). Let R = l i=1f i, for some nite elds F i. Let M be a faithful R-module. Let S = S i, where each S i is a subgroup of Fi. If f is a module-polynomial written as a sum of module-monomials, then it is decidable in polynomial time whether or not f = 0 is solvable with substitutions from S. In Chapter 7 we examine nonsolvable groups. The equation solvability problem over a non-solvable group is NP-complete [8, 9]. The following theorem is published in [2] in English, in [3] in Hungarian. Theorem (Theorem 7.1). The (polynomial) equivalence problem over a nite nonsolvable group is conp-complete. Some expressions can be much shorter if expressed not only by the basic operations of the algebra but by some new operations as well. For example, 6

7 the expression [[[x 1, x 2 ], x 3 ],..., x n ] has length n if the commutator is a basic operation, but has exponential length in n when expressed by only the group multiplication. Such a decrease in the length suggests that the complexities of the equivalence or equation solvability problems might change if the commutator (or another operation) is a basic operation. This motivates the denition of the extended problems in Chapter 8, when the input expression can contain new term operations expressed by the group multiplication. Denition (Denition 8.1). Let G = (G, ) be a nte group. We say that the extended (polynomial) equivalence (equation solvability) problem over G has polynomial time complexity if for arbitrary term expressions f 1,..., f k the (polynomial) equivalence (equation solvability) problem over (G,, f 1,..., f k ) has polynomial complexity. We say that the extended (polynomial) equivalence (equation solvability) problem over G is conp-complete (NP-complete) if there exist term expressions f 1,..., f k such that the (polynomial) equivalence (equation solvability) problem over (G,, f 1,..., f k ) is conp-complete (NP-complete). The main result of Chapter 8 is the dichotomy theorem for the equivalence and equation solvability problems over nite groups. Theorem (Theorem 8.2). Let G = (G, ) be a nite group. If G is nilpotent then the exteneded (polynomial) equivalence and the extended equation solvability problems over G have polynomial time complexities. If G is not nilpotent, then there exists a term f such that the (polynomial) equivalence over (G,, f) is conp-complete, and the equation solvability over (G,, f) is NP-complete. Most of the results discussed in the dissertation are not yet published. My publications in the topic are [2, 4, 5] in English and [1, 3] in Hungarian. 7

8 Publications in the topic of the dissertation [1] G. Horváth. Identities over nite groups (in Hungarian). Matematikai Lapok. New Series, 13 (2006/07)(2):1219, [2] G. Horváth, J. Lawrence, L. Mérai, and Cs. Szabó. The complexity of the equivalence problem for non-solvable groups. Bulletin of the London Mathematical Society, 39(3):433438, [3] G. Horváth and L. Mérai. The complexity of checking identities over non-solvable groups (in Hungarian). Matematikai Lapok. New Series, 13 (2006/07)(2):2027, [4] G. Horváth, C. L. Nehaniv, and Cs. Szabó. An assertion concerning functionally complete algebras and NP-completeness. Theoretical Computer Science, 407:591595, [5] G. Horváth and Cs. Szabó. The complexity of checking identities over nite groups. International Journal of Algebra Computation, 16(5): ,

9 Bibliography [6] S. Burris and J. Lawrence. The equivalence problem for nite rings. J. of Symb. Comp., 15:6771, [7] S. Burris and J. Lawrence. Results on the equivalence problem for nite groups. Alg. Univ., 52(4):495500, (2005). [8] M. Goldmann and A. Russell. The complexity of solving equations over nite groups. In Proceedings of the 14th Annual IEEE Conference on Computational Complexity, pages 8086, Atlanta, Georgia, [9] M. Goldmann and A. Russell. The complexity of solving equations over nite groups. Information and Computation, 178(253262), [10] H. Hunt and R. Stearns. The complexity for equivalence for commutative rings. Journal of Symbolic Computation, 10:411436, [11] O. Klíma. Complexity issues of checking identities in nite monoids. Semigroup Forum, 79(3):435444, [12] J. Lawrence and R. Willard. The complexity of solving polynomial equations over nite rings. manuscript, [13] P. Péladeau and D. Thérien. Sur les langages reconnus par des groupes nilpotents. C. R. Acad. Sci. Paris Sér. I Math, 306(2):9395, [14] Cs. Szabó and V. Vértesi. The complexity of checking identities for nite matrix rings. Algebra Universalis, 51:439445,

10 [15] Cs. Szabó and V. Vértesi. The complexity of the word-problem for nite matrix rings. Proceedings of the American Mathematical Society, 132: , [16] Cs. Szabó and V. Vértesi. The complexity of checking identities over nite rings. manuscript, [17] D. Thérien. Subword counting and nilpotent groups. In Combinatorics on words (Waterloo, Ont., 1982), pages Academic Press, Toronto, Ont.,

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