London Math Research Articles

Cycle Lengths Modulo k in Large 3-connected Cubic Graphs, Advances in Combinatorics

The existence of cycles with a given length is classical topic in graph theory with a plethora of open problems. Examples related to the main result of this paper include a conjecture of Burr and Erdős from 1976 asked whether for every integer $m$ and a positive odd integer $k$, there exists $d$ such that every graph with average degree at least $d$ contains a cycle of length $m$ modulo $k$; this conjecture was proven by Bollobás in [Bull. London Math. Soc. 9 (1977), 97-98]( https://doi.org/10.1112/blms/9.1.97). Another example is a problem of Erdős from the 1990s asking whether there exists $A\subseteq\mathbb{N}$ with zero density and constants $n_0$ and $d_0$ such that every graph with at least $n_0$ vertices and the average degree at least $d_0$ contains a cycle with length in the set $A$, which was resolved by Verstraete in [J. Graph Theory 49 (2005), 151-167]( https://doi.org/10.1002/jgt.20072). In 1983, Thomassen conjectured that for all integers $m$ and $k$, every graph with minimum degree $k+1$ contains a cycle of length $2m$ modulo $k$. Note that the parity condition in the first and the third conjectures is necessary because of bipartite graphs. The current paper contributes to this long line of research by proving that for every integer $m$ and a positive odd integer $k$, every sufficiently large $3$-connected cubic graph contains a cycle of length $m$ modulo $k$. The result is the best possible in the sense that the same conclusion is not true for $2$-connected cubic graphs or $3$-connected graphs with minimum degree three.

Large Book-Cycle Ramsey Numbers

Let $B_n^{(k)}$ be the book graph which consists of $n$ copies of $K_{k+1}$ all sharing a common $K_k$, and let $C_m$ be a cycle of length $m$. In this paper, we first determine the exact value of $r(B_n^{(2)}, C_m)$ for $\frac{8}{9}n+112\le m\le \lceil\frac{3n}{2}\rceil+1$ and $n \geq 1000$. This answers a question of Faudree, Rousseau, and Sheehan [Ars Combin., 31 (1991), pp. 239--248] in a stronger form when $m$ and $n$ are large. Building upon this exact result, we are able to determine the asymptotic value of $r(B_n^{(k)}, C_n)$ for each $k \geq 3$. Namely, we prove that for each $k \geq 3$, $r(B_n^{(k)}, C_n)= (k+1+o_k(1))n$. This extends a result due to Rousseau and Sheehan [J. London Math. Soc., 18 (1978), pp. 392--396].

An Erdős–Fuchs theorem for ordered representation functions

Let $k\geq 2$ be a positive integer. We study concentration results for the ordered representation functions $r^{\leq}_k(A,n) = \# \big\{ (a_1 \leq \dots \leq a_k) \in A^k : a_1+\dots+a_k = n \big\}$ and $r^{<}_k(A,n) = \# \big\{ (a_1 < \dots < a_k) \in A^k : a_1+\dots+a_k = n \big\}$ for any infinite set of non-negative integers $A$. Our main theorem is an Erd\H{o}s--Fuchs-type result for both functions: for any $c > 0$ and $\star \in \{\leq,<\}$ we show that $$\sum_{j = 0}^{n} \Big( r^{\star}_k(A,j) - c \Big) = o\big(n^{1/4} \log^{-1/2}n \big)$$ is not possible. We also show that the mean squared error $$E^\star_{k,c}(A,n)=\frac{1}{n} \sum_{j = 0}^{n} \Big( r^{\star}_k(A,j) - c \Big)^2$$ satisfies $\limsup_{n \to \infty} E^\star_{k,c}(A,n)>0$. These results extend two theorems for the non-ordered representation function proved by Erd\H{o}s and Fuchs in the case of $k=2$ (J. of the London Math. Society 1956).

Faithful real representations of groups of $F$-type

‎Groups of $F$-type were introduced in [B‎. ‎Fine and G‎. ‎Rosenberger‎, ‎Generalizing Algebraic Properties of Fuchsian Groups‎, emph{London Math. Soc. Lecture Note Ser.}, textbf{159} (1991)‎ ‎124--147.] as a natural algebraic generalization of Fuchsian groups‎. ‎They can be considered as the analogs of cyclically pinched one-relator groups where torsion is allowed‎. ‎Using the methods In [B‎. ‎Fine‎. ‎M‎. ‎Kreuzer and G‎. ‎Rosenberger‎, ‎Faithful Real Representations of Cyclically Pinched One-Relator Groups, Int. J. Group Theory, 3 ‎(2014) 1--8.] we prove that any hyperbolic group of $F$-type has a faithful representation in $PSL(2,mathbb R)$‎. ‎From this we also obtain that a cyclically pinched one-relator group has a faithful real representation if and only if it is hyperbolic‎. ‎We further survey the many nice properties of groups of $F$-type‎. ‎

Completeness in Quasi-Pseudometric Spaces—A Survey

The aim of this paper is to discuss the relations between various notions of sequential completeness and the corresponding notions of completeness by nets or by filters in the setting of quasi-metric spaces. We propose a new definition of right K-Cauchy net in a quasi-metric space for which the corresponding completeness is equivalent to the sequential completeness. In this way we complete some results of R. A. Stoltenberg, Proc. London Math. Soc. 17 (1967), 226–240, and V. Gregori and J. Ferrer, Proc. Lond. Math. Soc., III Ser., 49 (1984), 36. A discussion on nets defined over ordered or pre-ordered directed sets is also included.

The stabilizer of two-dimensional vector space of 27-dimensional module of type E6 over a field of characteristic two

The purpose of this paper is to use the notion of [Formula: see text]-sets (cocliques) introduced by the second author in [S. Aldhafeeri and M. Bani-Ata, On the construction of Lie-algebras of type [Formula: see text] for fields of characteristic two, Beitrag Zur Algebra und Geometry 58 (2017) 529–534.] and using Levi components and unipotent radical subgroups of [Formula: see text] to give an elementary and self-contained construction of the stabilizer of two dimensional vector space of 27-dimensional module of type [Formula: see text] over a field of characteristic two. This stabilizer is in fact the maximal parabolic subgroup [Formula: see text] of [Formula: see text] or a Borel subgroup. This construction is elementary on the account that we use not more than little naive linear algebra notions. For more information one can see [M. E. Aschbacher, The 27-dimensional module for [Formula: see text], 1, Invent. Math. 89 (1987) 159–195; M. E. Aschbacher, The 27-dimensional module for [Formula: see text], II, J. London Math. Soc. 37 (1988) 275–293; M. Bani-Ata, On Lie algebras of type [Formula: see text] and [Formula: see text] over finite fields of characteristic two, Preprint; B. Cooperstein, Subgroups of the group [Formula: see text] which are generated by root-subgroups, J. Algebra 46 (1977) 355–388.].

Periodic Intermediate β-Expansions of Pisot Numbers

The subshift of finite type property (also known as the Markov property) is ubiquitous in dynamical systems and the simplest and most widely studied class of dynamical systems are β -shifts, namely transformations of the form T β , α : x ↦ β x + α mod 1 acting on [ − α / ( β − 1 ) , ( 1 − α ) / ( β − 1 ) ] , where ( β , α ) ∈ Δ is fixed and where Δ ≔ { ( β , α ) ∈ R 2 : β ∈ ( 1 , 2 ) and 0 ≤ α ≤ 2 − β } . Recently, it was shown, by Li et al. (Proc. Amer. Math. Soc. 147(5): 2045–2055, 2019), that the set of ( β , α ) such that T β , α has the subshift of finite type property is dense in the parameter space Δ . Here, they proposed the following question. Given a fixed β ∈ ( 1 , 2 ) which is the n-th root of a Perron number, does there exists a dense set of α in the fiber { β } × ( 0 , 2 − β ) , so that T β , α has the subshift of finite type property? We answer this question in the positive for a class of Pisot numbers. Further, we investigate if this question holds true when replacing the subshift of finite type property by the sofic property (that is a factor of a subshift of finite type). In doing so we generalise, a classical result of Schmidt (Bull. London Math. Soc., 12(4): 269–278, 1980) from the case when α = 0 to the case when α ∈ ( 0 , 2 − β ) . That is, we examine the structure of the set of eventually periodic points of T β , α when β is a Pisot number and when β is the n-th root of a Pisot number.

Erratum

Erratum

Isomorphisms of subcategories of fusion systems of blocks and Clifford theory

Abstract Let k be an algebraically closed field of prime characteristic p. Let G be a finite group, let N be a normal subgroup of G, and let c be a G-stable block of kN so that ( k ⁢ N ) ⁢ c {(kN)c} is a p-permutation G-algebra. As in Section 8.6 of [M. Linckelmann, The Block Theory of finite Group Algebras: Volume 2, London Math. Soc. Stud. Texts 92, Cambridge University, Cambridge, 2018], a ( G , N , c ) {(G,N,c)} -Brauer pair ( R , f R ) {(R,f_{R})} consists of a p-subgroup R of G and a block f R {f_{R}} of ( k ⁢ C N ⁢ ( R ) ) {(kC_{N}(R))} . If Q is a defect group of c and f Q ∈ 𝐵 ⁢ ℓ ⁡ ( k ⁢ C N ⁢ ( Q ) ) {f_{Q}\in\operatorname{\textit{B}\ell}(kC_{N}(Q))} , then ( Q , f Q ) {(Q,f_{Q})} is a ( G , N , c ) {(G,N,c)} -Brauer pair. The ( G , N , c ) {(G,N,c)} -Brauer pairs form a (finite) poset. Set H = N G ⁢ ( Q , f Q ) {H=N_{G}(Q,f_{Q})} so that ( Q , f Q ) {(Q,f_{Q})} is an ( H , C N ⁢ ( Q ) , f Q ) {(H,C_{N}(Q),f_{Q})} -Brauer pair. We extend Lemma 8.6.4 of the above book to show that if ( U , f U ) {(U,f_{U})} is a maximal ( G , N , c ) {(G,N,c)} -Brauer pair containing ( Q , f Q ) {(Q,f_{Q})} , then ( U , f U ) {(U,f_{U})} is a maximal ( H , C N ⁢ ( c ) , f Q ) {(H,C_{N}(c),f_{Q})} -Brauer pair containing ( Q , f Q ) {(Q,f_{Q})} and conversely. Our main result shows that the subcategories of ℱ ( U , f U ) ⁢ ( G , N , c ) {\mathcal{F}_{(U,f_{U})}(G,N,c)} and ℱ ( U , f U ) ⁢ ( H , C N ⁢ ( Q ) , f Q ) {\mathcal{F}_{(U,f_{U})}(H,C_{N}(Q),f_{Q})} of objects between and including ( Q , f Q ) {(Q,f_{Q})} and ( U , f U ) {(U,f_{U})} are isomorphic. We close with an application to the Clifford theory of blocks.

On a problem of Sárközy and Sós for multivariate linear forms

We prove that for pairwise co-prime numbers $k\_1,\dots,k\_d \geq 2$ there does not exist any infinite set of positive integers $\mathcal{A}$ such that the representation function $r\_{\mathcal{A}}(n) = # { (a\_1, \dots, a\_d) {\in} \mathcal{A}^d : k\_1 a\_1 + \cdots + k\_d a\_d = n }$ becomes constant for $n$ large enough. This result is a particular case of our main theorem, which poses a further step towards answering a question of Sárközy and Sós and widely extends a previous result of Cilleruelo and Rué for bivariate linear forms (Bull. of the London Math. Society, 2009).

Zeta-functions of root systems and Poincaré polynomials of Weyl groups

We consider a certain linear combination of zeta-functions of root systems for a root system. Showing two different expressions of this linear combination, we find that a certain signed sum of zeta-functions of root systems is equal to a sum involving Bernoulli functions of root systems. This identity gives a non-trivial functional relation among zeta-functions of root systems, if the signed sum does not identically vanish. This is a generalization of the authors' previous result (Proc. London Math. Soc. 100 (2010), 303–347). We present several explicit examples of such functional relations. We give a criterion of the non-vanishing of the signed sum, in terms of Poincaré polynomials of associated Weyl groups. Moreover we prove a certain converse theorem.

When are full representations of algebras of operators on Banach spaces automatically faithful?

We examine the phenomenon when surjective algebra homomorphisms between algebras of operators on Banach spaces are automatically injective. In the first part of the paper we shall show that for certain Banach spaces $X$ the following property holds: For every non-zero Banach space $Y$ every surjective algebra homomorphism $\psi: \, \mathcal{B}(X) \rightarrow \mathcal{B}(Y)$ is automatically injective. In the second part of the paper we consider the question in the opposite direction: Building on the work of Kania, Koszmider and Laustsen \textit{(Trans. London Math. Soc., 2014)} we show that for every separable, reflexive Banach space $X$ there is a Banach space $Y_X$ and a surjective but not injective algebra homomorphism $\psi: \, \mathcal{B}(Y_X) \rightarrow \mathcal{B}(X)$.

Asymptotically Poincaré surfaces in quasi-Fuchsian manifolds

We introduce the notion of an asymptotically Poincaré family of surfaces in an end of a quasi-Fuchsian manifold. We show that any such family gives a foliation of an end by asymptotically parallel convex surfaces, and that the asymptotic behavior of the first and second fundamental forms determines the projective structure at infinity. As an application, we establish a conjecture of Labourie from [J. London Math. Soc. 45 (1992), pp. 549–565] regarding constant Gaussian curvature surfaces. We also derive consequences for constant mean curvature surfaces.

DIOPHANTINE APPROXIMATION WITH GAUSSIAN PRIMES

Abstract In this paper we prove that the exact analogue of the author’s work with real irrationals and rational primes (G. Harman, On the distribution of $\alpha p$ modulo one II, Proc. London Math. Soc. (3) 72, 1996, 241–260) holds for approximating $\alpha \in \mathbb{C}\setminus \mathbb{Q}[i]$ with Gaussian primes. To be precise, we show that for such $\alpha $ and arbitrary complex $\beta $ there are infinitely many solutions in Gaussian primes $p$ to $$\begin{equation*} ||\alpha p + \beta|| &lt;| p|^{-7/22}, \end{equation*}$$where $||\cdot ||$ denotes distance to a nearest member of $\mathbb{Z}[i]$. We shall, in fact, prove a slightly more general result with the Gaussian primes in sectors, and along the way improve a recent result due to Baier (S. Baier, Diophantine approximation on lines in $\mathbb{C}^2$ with Gaussian prime constraints, Eur. J. Math. 3, 2017, 614–649).

Universally Koszul and initially Koszul properties of Orlik–Solomon algebras

Let [Formula: see text] be a field with [Formula: see text] and [Formula: see text] an exterior algebra over [Formula: see text] with a standard grading [Formula: see text]. Let [Formula: see text] be a graded algebra, where [Formula: see text] is a graded ideal in [Formula: see text]. In this paper, we study universally Koszul and initially Koszul properties of [Formula: see text] and find classes of ideals [Formula: see text] which characterize such properties of [Formula: see text]. As applications, we classify arrangements whose Orlik–Solomon algebras are universally Koszul or initially Koszul. These results are related to a long-standing question of Shelton–Yuzvinsky [B. Shelton and S. Yuzvinsky, Koszul algebras from graphs and hyperplane arrangements, J. London Math. Soc. 56 (1997) 477–490].

A Constant-Time Algorithm for Middle Levels Gray Codes

For any integer $$n\ge 1$$, a middle levels Gray code is a cyclic listing of all n-element and $$(n+1)$$-element subsets of $$\{1,2,\ldots ,2n+1\}$$ such that any two consecutive sets differ in adding or removing a single element. The question whether such a Gray code exists for any $$n\ge 1$$ has been the subject of intensive research during the last 30 years, and has been answered affirmatively only recently [T. Mutze. Proof of the middle levels conjecture. Proc. London Math. Soc., 112(4):677–713, 2016]. In a follow-up paper [T. Mutze and J. Nummenpalo. An efficient algorithm for computing a middle levels Gray code. ACM Trans. Algorithms, 14(2):29 pp., 2018] this existence proof was turned into an algorithm that computes each new set in the Gray code in time $${{\mathcal {O}}}(n)$$ on average. In this work we present an algorithm for computing a middle levels Gray code in optimal time and space: each new set is generated in time $${{\mathcal {O}}}(1)$$ on average, and the required space is $${{\mathcal {O}}}(n)$$.

Two variations on ( A 3 × A 1 × A 1 ) (1) type discrete Painlevé equations

By considering the normalizers of reflection subgroups of types A (1) 1 and A (1) 3 in W ~ ( D 5 ( 1 ) ) , two subgroups: W ~ ( A 3 × A 1 ) ( 1 ) ⋉ W ( A 1 ( 1 ) ) and W ~ ( A 1 × A 1 ) ( 1 ) ⋉ W ( A 3 ( 1 ) ) can be constructed from a ( A 3 × A 1 × A 1 ) (1) type subroot system. These two symmetries arose in the studies of discrete Painlevé equations (Kajiwara K, Noumi M, Yamada Y. 2002 q -Painlevé systems arising from q -KP hierarchy. Lett. Math. Phys. 62 , 259–268; Takenawa T. 2003 Weyl group symmetry of type D (1) 5 in the q -Painlevé V equation. Funkcial. Ekvac. 46 , 173–186; Okubo N, Suzuki T. 2018 Generalized q -Painlevé VI systems of type ( A 2 n +1 + A 1 + A 1 ) (1) arising from cluster algebra. ( http://arxiv.org/abs/quant-ph/1810.03252 )), where certain non-translational elements of infinite order were shown to give rise to discrete Painlevé equations. We clarify the nature of these elements in terms of Brink-Howlett theory of normalizers of Coxeter groups (Howlett RB. 1980 Normalizers of parabolic subgroups of reflection groups. J. London Math. Soc. (2) 21 , 62–80; Brink B, Howlett RB. 1999 Normalizers of parabolic subgroups in Coxeter groups. Invent. Math. 136 , 323–351). This is the first of a series of studies which investigates the properties of discrete integrable equations via the theory of normalizers.

Conformal embedding and twisted theta functions at level one

In this paper, we consider the conformal embedding of s o ( r ) \mathfrak {so}(r) into s l ( r ) \mathfrak {sl}(r) and study relations between level one SO ⁡ ( r ) \operatorname {SO}(r) -theta functions and twisted SL ⁡ ( r ) \operatorname {SL}(r) -theta functions coming from parahoric moduli spaces. In particular, we give another proof of a theorem by Pauly-Ramanan [J. London Math. Soc. (2) 63 (2001), pp. 513–532].

The global dimension of the algebras of polynomial integro-differential operators 𝕀n and the Jacobian algebras 𝔸n

The aim of the paper is to prove two conjectures from the paper [V. V. Bavula, The algebra of integro-differential operators on a polynomial algebra, J. London Math. Soc. (2) 83 (2011) 517–543, arXiv:math.RA/0912.0723] that the (left and right) global dimension of the algebra [Formula: see text] of polynomial integro-differential operators and the Jacobian algebra [Formula: see text] is equal to [Formula: see text] (over a field of characteristic zero). The algebras [Formula: see text] and [Formula: see text] are neither left nor right Noetherian and [Formula: see text]. Furthermore, they contain infinite direct sums of nonzero left/right ideals and are not domains. An analogue of Hilbert’s Syzygy Theorem is proven for the algebras [Formula: see text], [Formula: see text] and their factor algebras. It is proven that the global dimension of all prime factor algebras of the algebras [Formula: see text] and [Formula: see text] is [Formula: see text] and the weak global dimension of all the factor algebras of [Formula: see text] and [Formula: see text] is [Formula: see text].

Jet modules for the centerless Virasoro-like algebra

In this paper, we studied the jet modules for the centerless Virasoro-like algebra which is the Lie algebra of the Lie group of the area-preserving diffeomorphisms of a [Formula: see text]-torus. The jet modules are certain natural modules over the Lie algebra of semi-direct product of the centerless Virasoro-like algebra and the Laurent polynomial algebra in two variables. We reduce the irreducible jet modules to the finite-dimensional irreducible modules over some infinite-dimensional Lie algebra and then characterize the irreducible jet modules with irreducible finite dimensional modules over [Formula: see text]. To determine the indecomposable jet modules, we use the technique of polynomial modules in the sense of [Irreducible representations for toroidal Lie algebras, J. Algebras 221 (1999) 188–231; Weight modules over exp-polynomial Lie algebras, J. Pure Appl. Algebra 191 (2004) 23–42]. Consequently, indecomposable jet modules are described using modules over the algebra [Formula: see text], which is the “positive part” of a Block type algebra studied first by [Some infinite-dimensional simple Lie algebras in characteristic [Formula: see text] related to those of Block, J. Pure Appl. Algebra 127(2) (1998) 153–165] and recently by [A [Formula: see text]-graded generalization of the Witt-algebra, preprint; Classification of simple Lie algebras on a lattice, Proc. London Math. Soc. 106(3) (2013) 508–564]).