Membership Problem Research Articles

On decidability of the product of subgroups membership problem for nilpotent groups

Abstract The product of subgroups membership problem for a finitely generated group 𝐺 is the decision problem, where, given two or more finitely generated subgroups K i K_{i} , i = 1 , … , m i=1,\ldots,m , of 𝐺 and a group element 𝑔, it is asked whether g ∈ ∏ i = 1 m K i g\in\prod_{i=1}^{m}K_{i} . In this paper, we prove that, for any finitely generated nilpotent group 𝑁 of class two, the product of two subgroups membership problem is decidable. Note that this problem is equivalent to the problem of determining the non-emptiness of the intersection of two cosets of finitely generated subgroups. We also show that, for a sufficiently large direct power H n \mathbb{H}^{n} of the Heisenberg group ℍ, there exists a product K = ∏ i = 1 4 K i K=\prod_{i=1}^{4}K_{i} of four finitely generated subgroups for which the membership problem is algorithmically undecidable. The proof of the last assertion is based on the undecidability of Hilbert’s 10th problem and interpretation of Diophantine equations in nilpotent groups. The question of decidability of the membership problem for a product of three finitely generated subgroups in a nilpotent group of class two remains open.

Translation-like actions by $\mathbb{Z}$, the subgroup membership problem, and Medvedev degrees of effective subshifts

We show that every infinite, locally finite, and connected graph admits a translation-like action by \mathbb{Z} , and that this action can be taken to be transitive exactly when the graph has either one or two ends. The actions constructed satisfy d(v,v\ast1)\leq3 for every vertex v . This strengthens a theorem by Brandon Seward. We also study the effective computability of translation-like actions on groups and graphs. We prove that every finitely generated infinite group with decidable word problem admits a translation-like action by \mathbb{Z} which is computable and satisfies an extra condition which we call decidable orbit membership problem. As a nontrivial application of our results, we prove that for every finitely generated infinite group with decidable word problem, effective subshifts attain all \Pi_{1}^{0} Medvedev degrees. This extends a classification proved by Joseph Miller for \mathbb{Z}^{d},\,d\geq1 .

On the power of local graph expansion grammars with and without additional restrictions

We study graph expansion grammars, a type of graph grammar that has recently been introduced with motivations in natural language processing. Graph expansion generalizes the well-known hyperedge replacement. In contrast to the latter, the former is able to generate graph languages of unbounded treewidth, like the set of all graphs. In an earlier paper, the complexity of the membership problem of the generated languages was studied, the main result being a polynomial parsing algorithm for local DAG expansion grammars (there called local DAG expansion grammars), a subclass of graph expansion grammars that generates directed acyclic graphs. Here, we study the generative power of local graph expansion grammars. While they, unrestricted, are able to simulate Turing machines, we identify natural restrictions that give rise to a pumping lemma and ensure that the generated languages have regular path languages as well as a semi-linear Parikh image.

Solving the membership problem for certain subgroups of [formula omitted]

For positive integers u and v, let Lu=[10u1] and Rv=[1v01]. Let Gu,v be the group generated by Lu and Rv. In a previous paper, the authors determined a characterization of matrices M=[abcd] in Gu,v when u,v≥3 in terms of the short continued fraction representation of b/d. We extend this result to the case where uv≥5. Additionally, we compute [Gu,v:Gu,v] where Gu,v={[1+uvn1vn2un31+uvn4]∈SL2(Z):(n1,n2,n3,n4)∈Z4} for u,v≥1, extending a result of Chorna, Geller, and Shpilrain and solving the membership problem for Gu,v with u,v≥1.

The central tree property and algorithmic problems on subgroups of free groups

Abstract We study the average case complexity of the Uniform Membership Problem for subgroups of free groups, and we show that it is orders of magnitude smaller than the worst case complexity of the best known algorithms. This applies to subgroups given by a fixed number of generators as well as to subgroups given by an exponential number of generators. The main idea behind this result is to exploit a generic property of tuples of words, called the central tree property. An application is given to the average case complexity of the Relative Primitivity Problem, using Shpilrain’s recent algorithm to decide primitivity, whose average case complexity is a constant depending only on the rank of the ambient free group.

The omega-reducibility of pseudovarieties of ordered monoids representing low levels of concatenation hierarchies

We deal with the question of the [Formula: see text]-reducibility of pseudovarieties of ordered monoids corresponding to levels of concatenation hierarchies of regular languages. A pseudovariety of ordered monoids [Formula: see text] is called [Formula: see text]-reducible if, given a finite ordered monoid M, for every inequality of pseudowords that is valid in [Formula: see text], there exists an inequality of [Formula: see text]-words that is also valid in [Formula: see text] and has the same “imprint” in M. Place and Zeitoun have recently proven the decidability of the membership problem for levels [Formula: see text], 1, [Formula: see text] and [Formula: see text] of concatenation hierarchies with level 0 being a finite Boolean algebra of regular languages closed under quotients. The solutions of these membership problems have been found by considering a more general problem of separation of regular languages and its further generalization — a problem of covering. Following the results of Place and Zeitoun, we prove that, for every concatenation hierarchy with level 0 being represented by a locally finite pseudovariety of monoids, the pseudovarieties corresponding to levels [Formula: see text] and [Formula: see text] are [Formula: see text]-reducible. As a corollary of these results, we obtain that, for every concatenation hierarchy with level 0 being represented by a locally finite pseudovariety of monoids, the pseudovarieties corresponding to levels [Formula: see text] and [Formula: see text] are definable by [Formula: see text]-inequalities. Furthermore, in the special case of the Straubing–Thérien hierarchy, using a characterization theorem for level 2 by Place and Zeitoun, we obtain that the level 2 is definable by [Formula: see text]-identities.

All about unambiguous polynomial closure

We study a standard operator on classes of languages: unambiguous polynomial closure. We prove that for every class C of regular languages satisfying mild properties, the membership problem for its unambiguous polynomial closure UPol(C) reduces to the same problem for C. We also show that unambiguous polynomial closure coincides with alternating left and right deterministic closure. Moreover, we prove that if additionally C is finite, the separation and covering problems are decidable for UPol(C). Finally, we present an overview of the generic logical characterizations of the classes built using unambiguous polynomial closure.

The membership problem for subsemigroups of [formula omitted] is NP-complete

We show that the problem of determining if the identity matrix belongs to a finitely generated semigroup of 2×2 matrices from the modular group PSL2(Z), the Special Linear group SL2(Z) and the General Linear Group GL2(Z) is solvable in NP. We extend this to prove that the membership problem is decidable in NP for GL2(Z) and for any arbitrary regular expression over matrices from SL2(Z). We then derive that the problems of whether a given finite set of matrices from SL2(Z) or PSL2(Z) generates a group or a free semigroup are both decidable in NP. The previous algorithm for these problems, shown in 2005 by Choffrut and Karhumäki, was in EXPSPACE. Our algorithm is based on new techniques allowing us to operate on compressed word representations of matrices without explicit expansions. When combined with the known NP-hard lower bound, this proves that the identity (and thus membership) problem over GL2(Z) is NP-complete, and the group problem and the non-freeness problem in SL2(Z) are NP-complete. Thus the paper answer the long standing open question on the complexity of the membership problem in semigroups generated by matrices from GL2(Z). We develop novel techniques that can be used for solving numerical matrix problems in symbolic form, which are applicable for solving compressed word problems for groups and semigroups, bridging the gap between combinatorial group theory, computational problems on matrices and complexity theory.1

ChainedFilter: Combining Membership Filters by Chain Rule

Membership (membership query/membership testing) is a fundamental problem across databases, networks and security. However, previous research has primarily focused on either approximate solutions, such as Bloom Filters, or exact methods, like perfect hashing and dictionaries, without attempting to develop an integral theory. In this paper, we propose a unified and complete theory, namely chain rule, for general membership problems, which encompasses both approximate and exact membership as extreme cases. Building upon the chain rule, we introduce a straightforward yet versatile algorithm framework, namely ChainedFilter, to combine different elementary filters without losing information. Our evaluation results demonstrate that ChainedFilter improves performance of many applications including static dictionary, lossless data compression, Cuckoo Hashing, LSM-Tree and Learned Filters.

Undecidability of admissibility in the product of two Alt logics

Abstract The product of two $\textbf {Alt}$ logics possesses the polynomial product finite model property and its membership problem is $\textbf {coNP}$-complete. Using a reduction from an undecidable domino-tiling problem, we prove that its admissibility problem is undecidable.

Transduction from trees to graphs through folding

We introduce a fold operation that realises a tree-to-graph transduction by merging selected nodes in the input tree to form a possibly cyclic output graph. The work is motivated by the increasing use of graph-based representations in semantic parsing. We show that a suitable class of graphs languages can be generated by applying the fold operation to regular unranked tree languages. We investigate two versions of the fold operation, one that preserves a depth-first ordering between the edges, and one that does not. Finally, we demonstrate that the time complexity for the associated non-uniform membership problem is solvable in polynomial time for the order-preserving version, and NP-complete for the order-cancelling one.

Complexity Column

Algorithmic problems in groups and semigroups are a classic research area that has attracted quite a bit of attention since the seminal works by Markov and others. These results demonstrated that very natural questions such as the Group Membership problem can be undecidable even in groups of matrices, and even in decidable cases can be hard. Moreover, the exact complexity of such problems can be difficult to determine. The collection of computational problems being studied is rich and include, apart from the Group and Semigroup Membership problems, the Word problem, the Intersection problem for subgroups and subsemigroups, and many more.

Membership function‐dependent predictive control for interval type‐2 Takagi–Sugeno fuzzy system

AbstractIn this paper, the problem of membership function‐dependent model predictive control (MFD‐MPC) of interval type‐2 Takagi–Sugeno (IT2 T‐S) fuzzy system is investigated. An online MFD‐MPC optimization problem is formulated to minimize an infinite horizon objective function subject to hard constraints on control input and system state. By using the variable dimension expansion method and dividing the operation domain of independent variables into several subdomains, a subregion‐based state feedback model predictive controller that offers more design flexibility is designed to stabilize the open‐loop fuzzy system. Based on descriptor system theory, the variable mismatching phenomenon between MFs and the coupling between plant, controller, and Lyapunov matrix are eliminated. Then, using the piecewise linear approximation technology, MFD sufficient conditions of low conservatism on guaranteeing recursive feasibility and the stability of IT2 closed‐loop fuzzy system are deduced by tightening terminal constraint set. Finally, a numerical example is provided to further explain the advance of the proposed method.

On the complexity of decision problems for some classes of machines and applications

We study the computational complexity of important decision problems — including general membership, fixed-machine membership, emptiness, disjointness, equivalence, containment, universe, and finiteness problems — for various restrictions and combinations of two-way nondeterministic reversal-bounded multicounter machines (2NCM) and two-way pushdown automata. We show that the general membership problem (respectively fixed membership problem) for 2NCM is NP-complete (respectively in P).We then give applications to some problems in coding theory. We examine generalizations of various types of codes with marginal errors. For example, a language L is k-infix-free if there is no non-empty string y in L that is an infix of more than k strings in L−{y}. Our general results imply the complexity of determining whether a given machine accepts a k-infix-free language, for one- and two-way deterministic and nondeterministic finite automata (answering an open question from the literature).

Segre-driven radicality testing

We present a probabilistic algorithm to test if a homogeneous polynomial ideal I defining a scheme X in Pn is radical using Segre classes and other geometric notions from intersection theory which is applicable for certain classes of ideals. If all isolated primary components of the scheme X are reduced and it has no embedded components outside of the singular locus of Xred=V(I), then the algorithm is not applicable and will return that it is unable to decide radically; in all the other cases it will terminate successfully and in either case its complexity is singly exponential in n. The realm of the ideals for which our radical testing procedure is applicable and for which it requires only single exponential time includes examples which are often considered pathological, such as the ones drawn from the famous Mayr-Meyer set of ideals which exhibit doubly exponential complexity for the ideal membership problem.

Group-like small cancellation theory for rings

In this paper, we develop a small cancellation theory for associative algebras with a basis of invertible elements. Namely, we study quotients of a group algebra of a free group and introduce three axioms for the corresponding defining relations. We show that the obtained ring is non-trivial. Moreover, we show that this ring enjoys a global filtration that agrees with relations, find a basis of the ring as a linear space and establish the corresponding structure theorems. We also provide a revision of a concept of Gröbner basis for our rings and establish a greedy algorithm for the Ideal Membership Problem.

Many bounded versions of undecidable problems are NP-hard

Several physically inspired problems have been proven undecidable; examples are the spectral gap problem and the membership problem for quantum correlations. Most of these results rely on reductions from a handful of undecidable problems, such as the halting problem, the tiling problem, the Post correspondence problem or the matrix mortality problem. All these problems have a common property: they have an NP-hard bounded version. This work establishes a relation between undecidable unbounded problems and their bounded NP-hard versions. Specifically, we show that NP-hardness of a bounded version follows easily from the reduction of the unbounded problems. This leads to new and simpler proofs of the NP-hardness of bounded version of the Post correspondence problem, the matrix mortality problem, the positivity of matrix product operators, the reachability problem, the tiling problem, and the ground state energy problem. This work sheds light on the intractability of problems in theoretical physics and on the computational consequences of bounding a parameter.

Post-Quantum Signature Scheme Based on the Root Extraction Problem over Mihailova Subgroups of Braid Groups

In this paper, by introducing an isomorphism from the Mihailova subgroup of F2×F2 to the Mihailova subgroups of a braid group, we give an explicit presentation of Mihailova subgroups of a braid group. Hence, in a braid group, there are some Mihailova subgroups experiencing unsolvable subgroup membership problem. Based on this, we propose a post-quantum signature scheme of the Wang–Hu scheme, and we show that the signature scheme is free of quantum computational attack.

On the word problem for weakly compressible monoids

We study the language-theoretic properties of the word problem, in the sense of Duncan & Gilman, of weakly compressible monoids, as defined by Adian & Oganesian. We show that if C is a reversal-closed super-, as defined by Greibach, then M has word problem in C if and only if its compressed left monoid L(M) has word problem in C. As a special case, we may take C to be the class of context-free or indexed languages. As a corollary, we find many new classes of monoids with decidable rational subset membership problem. Finally, we show that it is decidable whether a one-relation monoid containing a non-trivial idempotent has context-free word problem. This answers a generalization of a question first asked by Zhang in 1992.

The fully compressed subgroup membership problem

Suppose that F is a free group and k is a natural number. We show that the fully compressed membership problem for k-generated subgroups of F is solvable in polynomial time. In order to do this, we adapt the theory of Stallings' foldings to handle edges with compressed labels. This partially answers a question of Markus Lohrey.