Quotient Complex Research Articles

Hyperbolic quotients of projection complexes

This paper is a continuation of our previous work with Margalit where we studied group actions on projection complexes. In that paper, we demonstrated sufficient conditions so that the normal closure of a family of subgroups of vertex stabilizers is a free product of certain conjugates of these subgroups. In this paper, we study both the quotient of the projection complex by this normal subgroup and the action of the quotient group on the quotient of the projection complex. We show that under certain conditions the quotient complex is $\\delta$-hyperbolic. Additionally, under certain circumstances, we show that if the original action on the projection complex was a non-elementary WPD action, then so is the action of the quotient group on the quotient of the projection complex. This implies that the quotient group is acylindrically hyperbolic.

A counter-example to the Schenck-Stiller “2r + 1” conjecture

Let Δ be a connected, pure 2-dimensional simplicial complex embedded in R2 and let Cr(Δˆ) be the homogenized spline module of Δ with smoothness r as in [7]. To study Cr(Δˆ), Schenck and Stillman developed in [7] the quotient complex S•/J•. In [8], Schenck and Stiller conjectured that the regularity of H1(S•/J•) is less than 2r+1. In this article, we pose a counterexample to this “2r+1” conjecture. We also propose some modifications to the conjecture.

Complexity of suffix-free regular languages

We study various complexity properties of suffix-free regular languages. A sequence (Lk,Lk+1,…) of regular languages in some class, where n is the quotient complexity of Ln, is most complex if its languages Ln meet the complexity upper bounds for all basic measures. It is known that there exist such most complex sequences in several classes of regular languages. In contrast to this, we prove that there does not exist a most complex sequence in the class of suffix-free regular languages. However, we do exhibit two such sequences that together meet upper bounds for all basic measures.

Most Complex Regular Ideal Languages

A right ideal (left ideal, two-sided ideal) is a non-empty language $L$ over an alphabet $\Sigma$ such that $L=L\Sigma^*$ ($L=\Sigma^*L$, $L=\Sigma^*L\Sigma^*$). Let $k=3$ for right ideals, 4 for left ideals and 5 for two-sided ideals. We show that there exist sequences ($L_n \mid n \ge k $) of right, left, and two-sided regular ideals, where $L_n$ has quotient complexity (state complexity) $n$, such that $L_n$ is most complex in its class under the following measures of complexity: the size of the syntactic semigroup, the quotient complexities of the left quotients of $L_n$, the number of atoms (intersections of complemented and uncomplemented left quotients), the quotient complexities of the atoms, and the quotient complexities of reversal, star, product (concatenation), and all binary boolean operations. In that sense, these ideals are "most complex" languages in their classes, or "universal witnesses" to the complexity of the various operations.

The Symmetric Commutator Homology of Link Towers and Homotopy Groups of 3-Manifolds

A link tower is a sequence of links with the structure given by removing the last components. Given a link tower, we prove that there is a chain complex consisting of (non-abelian) groups given by the symmetric commutator subgroup of the normal closures in the link group of the meridians excluding the meridian of the last component with the differential induced by removing the last component. Moreover, the homology groups of these naturally constructed chain complexes are isomorphic to the homotopy groups of the manifold M under certain hypothesis. These chain complexes have canonical quotient abelian chain complexes in Minor’s homotopy link groups with their homologies detecting certain differences of the homotopy link groups in the towers.

Riemann Hypothesis and strongly Ramanujan complexes from GLn

We investigate the Riemann Hypothesis on combinatorial zeta functions associated to finite quotients of the affine building of GLn. We prove that if the quotient complex is strongly Ramanujan then these zeta functions satisfy the Riemann Hypothesis. On the other hand, we show that the converse statement is also true provided the extra generic condition. In the end, we give an example to show that this generic condition is indeed necessary.

Asymptotic Approximation for the Quotient Complexities of Atoms

In a series of papers, Brzozowski together with Tamm, Davies, and Szykuła studied the quotient complexities of atoms of regular languages [6, 7, 3, 4]. The authors obtained precise bounds in terms of binomial sums for the most complex situations in the following five cases: (G): general, (R): right ideals, (L): left ideals, (T): two-sided ideals and (S): suffix-free languages. In each case let κc(n) be the maximal complexity of an atom of a regular language L, where L has complexity n ≥ 2 and belongs to the class C ϵ {G, R, L, T , S}. It is known that κT(n) ≤ κL(n) = κR(n) ≤ κG(n) < 3n and κS(n) = κL(n−1). We show that the ratio κC(n)/κC(n−1) tends exponentially fast to 3 in all five cases but it remains different from 3. This behaviour was suggested by experimental results of Brzozowski and Tamm; and the result for G was shown independently by Luke Schaeffer and the first author soon after the paper of Brzozowski and Tamm appeared in 2012. However, proofs for the asymptotic behavior of κG(n)/κG(n−1) were never published; and the results here are valid for all five classes above. Moreover, there is an interesting oscillation for all C: for almost all n we have κC(n)/κC(n−1) > 3 if and only if κC(n+1)/κC(n) < 3.

Quotient Complexities of Atoms in Regular Ideal Languages

A (left) quotient of a language L by a word w is the language w −1L = {x | wx ϵ L}. The quotient complexity of a regular language L is the number of quotients of L; it is equal to the state complexity of L, which is the number of states in a minimal deterministic finite automaton accepting L. An atom of L is an equivalence class of the relation in which two words are equivalent if for each quotient, they either are both in the quotient or both not in it; hence it is a non-empty intersection of complemented and uncomplemented quotients of L. A right (respectively, left and two-sided) ideal is a language L over an alphabet Σ that satisfies L = LΣ* (respectively, L = Σ*L and L = Σ*LΣ*). We compute the maximal number of atoms and the maximal quotient complexities of atoms of right, left and two-sided regular ideals.

Maximally Atomic Languages

The atoms of a regular language are non-empty intersections of complemented and uncomplemented quotients of the language. Tight upper bounds on the number of atoms of a language and on the quotient complexities of atoms are known. We introduce a new class of regular languages, called the maximally atomic languages, consisting of all languages meeting these bounds. We prove the following result: If L is a regular language of quotient complexity n and G is the subgroup of permutations in the transition semigroup T of the minimal DFA of L, then L is maximally atomic if and only if G is transitive on k-subsets of 1,...,n for 0 <= k <= n and T contains a transformation of rank n-1.

Quotient Complexity of Bifix-, Factor-, and Subword-free Regular Language

A language L is prefix-free if whenever words u and v are in L and u is a prefix of v, then u = v. Suffix-, factor-, and subword-free languages are defined similarly, where by "subword" we mean "subsequence", and a language is bifix-free if it is both prefix- and suffix-free. These languages have important applications in coding theory. The quotient complexity of an operation on regular languages is defined as the number of left quotients of the result of the operation as a function of the numbers of left quotients of the operands. The quotient complexity of a regular language is the same as its state complexity, which is the number of states in the complete minimal deterministic finite automaton accepting the language. The state/quotient complexity of operations in the classes of prefix- and suffix-free languages has been studied before. Here, we study the complexity of operations in the classes of bifix-, factor-, and subword-free languages. We find tight upper bounds on the quotient complexity of intersection, union, difference, symmetric difference, concatenation, star, and reversal in these three classes of languages.

Quotient Complexity of Closed Languages

A language L is prefix-closed if, whenever a word w is in L, then every prefix of w is also in L. We define suffix-, factor-, and subword-closed languages in an analogous way, where by factor we mean contiguous subsequence, and by subword we mean scattered subsequence. We study the state complexity (which we prefer to call quotient complexity) of operations on prefix-, suffix-, factor-, and subword-closed languages. We find tight upper bounds on the complexity of the subword-closure of arbitrary languages, and on the complexity of boolean operations, concatenation, star, and reversal in each of the four classes of closed languages. We show that repeated applications of positive closure and complement to a closed language result in at most four distinct languages, while Kleene closure and complement give at most eight.

COMPLEXITY OF ATOMS OF REGULAR LANGUAGES

The quotient complexity of a regular language L, which is the same as its state complexity, is the number of left quotients of L. An atom of a non-empty regular language L with n quotients is a non-empty intersection of the n quotients, which can be uncomplemented or complemented. An NFA is atomic if the right language of every state is a union of atoms. We characterize all reduced atomic NFAs of a given language, i.e., those NFAs that have no equivalent states. We prove that, for any language L with quotient complexity n, the quotient complexity of any atom of L with r complemented quotients has an upper bound of 2n − 1 if r = 0 or r = n; for 1 ≤ r ≤ n − 1 the bound is[Formula: see text] For each n ≥ 2, we exhibit a language with 2n atoms which meet these bounds.

Quotient complexity of ideal languages

A language L over an alphabet Σ is a right (left) ideal if it satisfies L=LΣ∗ (L=Σ∗L). It is a two-sided ideal if L=Σ∗LΣ∗, and an all-sided ideal if L=Σ∗▪L, the shuffle of Σ∗ with L. Ideal languages are not only of interest from the theoretical point of view, but also have applications to pattern matching. We study the state complexity of common operations in the class of regular ideal languages, but prefer to use the equivalent term “quotient complexity”, which is the number of distinct left quotients of a language. We find tight upper bounds on the complexity of each type of ideal language in terms of the complexity of an arbitrary generator and of the minimal generator, and also on the complexity of the minimal generator in terms of the complexity of the language. Moreover, tight upper bounds on the complexity of union, intersection, set difference, symmetric difference, concatenation, star, and reversal of ideal languages are derived.

QUOTIENT COMPLEXITY OF STAR-FREE LANGUAGES

The quotient complexity, also known as state complexity, of a regular language is the number of distinct left quotients of the language. The quotient complexity of an operation is the maximal quotient complexity of the language resulting from the operation, as a function of the quotient complexities of the operands. The class of star-free languages is the smallest class containing the finite languages and closed under boolean operations and concatenation. We prove that the tight bounds on the quotient complexities of union, intersection, difference, symmetric difference, concatenation and star for star-free languages are the same as those for regular languages, with some small exceptions, whereas 2n-1 is a lower bound for reversal.

DUALITY IN FILTERED FLOER–NOVIKOV COMPLEXES

We prove that a certain bilinear pairing (analogous to the Poincaré–Lefschetz intersection pairing) between filtered sub- and quotient complexes of a Floer-type chain complex and of its "opposite complex" is always nondegenerate on homology. This implies a duality relation for the Oh–Schwarz-type spectral invariants of these complexes which (in Hamiltonian Floer theory) was established in the special case that the period map has discrete image by Entov and Polterovich. The duality relation served as a key lemma in Entov and Polterovich's construction of a Calabi quasimorphism on certain rational symplectic manifolds, and the result that we prove here implies that their construction remains valid when the rationality hypothesis is dropped. Apart from this, we also use the nondegeneracy of the pairing to establish a new formula for what we have previously called the boundary depth of a Floer chain complex; this formula shows that the boundary depth is unchanged under passing to the opposite complex.

Equivariant bundles and isotropy representations

We introduce a new construction, the isotropy groupoid , to organize the orbit data for split Γ-spaces. We show that equivariant principal G-bundles over split Γ-CW complexes X can be effectively classified by means of representations of their isotropy groupoids. For instance, if the quotient complex A = Γ \ X is a graph, with all edge stabilizers toral subgroups of Γ, we obtain a purely combinatorial classification of bundles with structural group G a compact connected Lie group. If G is abelian, our approach gives combinatorial and geometric descriptions of some results of Lashof–May–Segal [18] and Goresky–Kottwitz–MacPherson [10].

Quotient Complexity of Regular Languages

The past research on the state complexity of operations on regular languages is examined, and a new approach based on an old method (derivatives of regular expressions) is presented. Since state complexity is a property of a language, it is appropriate to define it in formal-language terms as the number of distinct quotients of the language, and to call it "quotient complexity". The problem of finding the quotient complexity of a language f(K,L) is considered, where K and L are regular languages and f is a regular operation, for example, union or concatenation. Since quotients can be represented by derivatives, one can find a formula for the typical quotient of f(K,L) in terms of the quotients of K and L. To obtain an upper bound on the number of quotients of f(K,L) all one has to do is count how many such quotients are possible, and this makes automaton constructions unnecessary. The advantages of this point of view are illustrated by many examples. Moreover, new general observations are presented to help in the estimation of the upper bounds on quotient complexity of regular operations.

A low pretransplant peripheral blood mononuclear cell complex I activity predicts metabolic disturbances and inability to regain fat free mass in cirrhotic patients undergoing liver transplantation

Liver cirrhosis is associated with malnutrition and often, after liver transplantation, with the development of obesity and the inability to gain lean body mass. We have previously shown that peripheral blood mononuclear cell (PBMNC) complex I activity could be an appropriate marker for nutritional assessment. In this context, we hypothesized that a low pretransplant PBMNC complex I activity may predict a poor nutritional status in cirrhotic patients undergoing liver transplantation. Fifteen cirrhotic patients (CP) (8 men and 7 women) were recruited and investigated before and 4 months after liver transplantation. Body weight, body composition by DEXA, anthropometric measures (triceps skinfold thickness and midarm muscle circumference), resting energy expenditure, respiratory quotient and PBMNC complex I activity were measured on both time points. Patients were divided into 2 groups depending on their pretransplant PBMNC complex I activity (low vs high complex I activity [CP(low CI) vs CP(high CI)]), using as an arbitrary cutoff value-the mean complex I activity observed in age-matched healthy controls. Before transplantation, the CP(low CI) group who showed a lower complex I activity (2.11 +/- 0.53 vs 4.54 +/- 0.98 nmol/min per milligram of protein, P < .01) was significantly younger (44 +/- 9 vs 62 +/- 8 years old, P < .01); no differences were observed for any other nutritional parameters when compared to the CP(high CI) group. After transplantation, only the CP(low CI) group demonstrated a significant increase of complex I activity (+77%, P < .01), respiratory quotient (+10.5%, P < .02), triceps skinfold thickness (+126%, P < .005), and a significant decrease of fat-free mass (-8%, P < .01). In summary, our findings indicate that a low pretransplant PBMNC complex I activity in cirrhotic patients could be a useful marker of poor nutritional status despite the lack of traditional indicators of malnutrition by predicting metabolic disturbances and an inability to regain fat-free mass after liver transplantation.

The structure and singularities of quotient arc complexes

A well-known combinatorial fact is that the simplicial complex consisting of disjointly embedded chords in a convex planar polygon is a sphere. For any surface F with non-empty boundary, there is an analogous complex QA(F) consisting of equivalence classes of arcs in F connecting a given finite set of points in its boundary modulo diffeomorphisms of F pointwise fixing the boundary and any punctures. The main result of this paper is the determination of those complexes QA(F) that are also spheres. This classification has consequences for Riemann's moduli space of curves via its known identification with a related quotient arc complex in the punctured case with no boundary. Namely, the essential singularities of the natural cellular compactification of Riemann's moduli space can be described.

Teichmüller spaces for pointed Fuchsian groups and their modular groups

It is known that the modular group Mod X acts discontinuously (but not freely) on the Teichmüller space T ( X ) for a finite type Riemann surface X , while it does not necessarily act discontinuously on T ( X ) when X is of infinite type. The primary purpose of the paper is to discuss those subgroups of Mod X acting discontinuously and freely on T ( X ) and to discuss the properties of the corresponding quotient complex manifolds as well. Actually, we will discuss some generalized Teichmüller spaces, the Teichmüller spaces for pointed Riemann surfaces and pointed Fuchsian groups, and their modular groups, generalizing and completing some results of Bers [Be1], Kra [Kr1] and Nag ([Na1], [Na3], [Na4]).