Homomorphisms from Automorphism Groups of Free Groups

The automorphism group of a finitely generated free group isthe normal closure of a single element of order 2. If m <n, then a homomorphism Aut(F_n)Aut(F_m) can have image of cardinalityat most 2. More generally, this is true of homomorphisms fromAut(F_n) to any group that does not contain an isomorphic imageof the symmetric group S_n+1. Strong restrictions are also obtainedon maps to groups that do not contain a copy of W_n = (Z/2)ⁿ S_n, or of Z^n–1. These results place constraints on howAut(F_n) can act. For example, if n 3, any action of Aut(F_n)on the circle (by homeomorphisms) factors through det : Aut(F_n)Z₂.2000 Mathematics Subject Classification 20F65, 20F28 (primary).     

The Non-Finite Presentability of the Automorphism Group of the Free Z-Group of Rank Two

Let G be a group, and let F_n[G] be the free G-group of rankn. Then F_n[G] is just the natural non-abelian analogue of thefree ZG-module of rank n, and correspondingly the group _n(G)of equivariant automorphisms of F_n[G] is a natural analogueof the general linear group GL_n(ZG). The groups _n(G) have beenstudied recently in [4, 8, 5]. In particular, in [5] it wasshown that if G is not finitely presentable (f.p.) then neitheris _n(G), and conversely, that _n(G) is f.p. if G is f.p. andn2. It is a common phenomenon that for small ranks the automorphismgroups of free objects may behave unstably (see the survey article[2]), and the main aim of the present paper is to show thatthis turns out to be the case for the groups ₂(G).     

Automorphisms of Relatively Free Groups in the Variety N2A {wedge} AN2 {wedge} Nc

For positive integers n and c, with n 2, let G_{n, c} be a relativelyfree group of finite rank n in the variety N₂A AN₂ N_c. Itis shown that the subgroup of the automorphism group Aut(G_n,c) of G_{n, c} generated by the tame automorphisms and an explicitlydescribed finite set of IA-automorphisms of G_{n, c} has finiteindex in Aut(G_{n, c}). Furthermore, it is proved that there areno non-trivial elements of G_{n, c} fixed by every tame automorphismof G_{n, c}.     

Complex Dynamics of Convergence Acceleration

Generalized Steffensen methods are nonderivative algorithmsfor the computation of fixed points of a function f. They replacethe functional iteration Z_m+1=f(Z_m) with Z_m+1=F_n(Z_m, where F_nis explicitly provided for every n 1 as a quotient of two Hankeldeterminants. In this paper we derive rules pertaining to thelocal behaviour of these methods. Specifically, and subjectto analyticity, given that is a bounded fixed point of f, thenit is also a fixed point of F_n. Moreover, unless f'() vanishesor is a root of unity, becomes a superattractive fixed pointof F_n of degree n; if f'() is a root of unity of minimal degreeq2, then is (as a fixed point of F_n) superattractive of degreemin {q-1, n}; if f'()=1, then is attractive for F_n; and, finally,if is superattractive of degree s (as a fixed point of f),then it becomes superattractive of degree (s + 1)^n–1(ns+ s + 1)–1. Attractivity rules change at infinity (providedthat f()=). Broadly speaking, infinity becomes less attractivefor F_n, Since one is interested in convergence to finite fixedpoints, this further enhances the appeal of generalized Steffensenmethods.     

Fields of Definition for Division Algebras

Let A be a finite-dimensional division algebra containing abase field k in its center F. A is defined over a subfield F₀if there exists an F₀-algebra A₀ such that . The following are shown. (i) In many cases A canbe defined over a rational extension of k. (ii) If A has odddegree n 5, then A is defined over a field F₀ of transcendencedegree 1/2(n–1)(n–2) over k. (iii) If A is a Z/mx Z/2-crossed product for some m 2 (and in particular, if Ais any algebra of degree 4) then A is Brauer equivalent to atensor product of two symbol algebras. Consequently, M_m(A) canbe defined over a field F₀ such that trdeg_k(F₀) 4. (iv) IfA has degree 4 then the trace form of A can be defined overa field F₀ of transcendence degree 4. (In (i), (iii) and (iv)it is assumed that the center of A contains certain roots ofunity.)     

Connectedness and Stability of Julia Sets of the Composition of Polynomials of the Form z2+cn

For a sequence (c_n) of complex numbers, the quadratic polynomialsf_cn(z) := z² + c_n and the sequence (F_n) of iterates F_n := f_cn...f_c1areconsidered. The Fatou set F_(cn) is by definition the set ofall z C^ such that (F_n) is normal in some neighbourhood ofz, while the complement of F_(cn) is called the Julia set J_(cn).The aim of this article is to study the connectedness and stabilityof the Julia set J_(cn) provided that the sequence (c_n) is bounded.     

Normal Transversality and Uniform Bounds

Let A be a commutative ring. A graded A-algebra U = _n0 U_n isa standard A-algebra if U₀ = A and U = A[U₁] is generated asan A-algebra by the elements of U₁. A graded U-module F = _n0F_nis a standard U-module if F is generated as a U-module by theelements of F₀, that is, F_n = U_nF₀ for all n 0. In particular,F_n = U₁F_n–1 for all n 1. Given I, J, two ideals of A,we consider the following standard algebras: the Rees algebraof I, R(I) = _n0Iⁿtⁿ = A[It] A[t], and the multi-Rees algebraof I and J, R(I, J) = _n0(_p+q=nI^pJ^qu^pv^q) = A[Iu, Jv] A[u, v].Consider the associated graded ring of I, G(I) = R(I) A/I =_n0Iⁿ/Iⁿ⁺¹, and the multi-associated graded ring of I and J,G(I, J) = R(I, J) A/(I+J) = _n0(_p+q=nI^pJ^q/(I+J)I^pJ^q). We canalways consider the tensor product of two standard A-algebrasU = _p0U_p and V = _q0V_q as a standard A-algebra with the naturalgrading U V = _n0(_p+q=nU_p V_q). If M is an A-module, we havethe standard modules: the Rees module of I with respect to M,R(I; M) = _n0IⁿMtⁿ = M[It] M[t] (a standard R(I)-module), andthe multi-Rees module of I and J with respect to M, R(I, J;M) = _n0(_p+q=nI^pJ^qMu^pv^q) = M[Iu, Jv] M[u, v] (a standard R(I,J)-module). Consider the associated graded module of M withrespect to I, G(I; M) = R(I; M) A/I = _n0IⁿM/Iⁿ⁺¹M (a standardG(I)-module), and the multi-associated graded module of M withrespect to I and J, G(I, J; M) = R(I, J; M) A/(I+J) = _n0(_p+q=nI^pJ^qM/(I+J)I^pJ^qM)(a standard G(I, J)-module). If U, V are two standard A-algebras,F is a standard U-module and G is a standard V-module, thenF G = _n0(_p+q=nF_p G_q) is a standard U V-module. Denote by :R(I) R(J; M) R(I, J; M) and :R(I, J; M) R(I+J;M) the natural surjective graded morphisms of standard RI) R(J)-modules. Let :R(I) R(J; M) R(I+J; M) be . Denote by :G(I) G(J; M) G(I, J; M) and :G(I, J; M) G(I+J; M) the tensor productof and by A/(I+J); these are two natural surjective gradedmorphisms of standard G(I) G(J)-modules. Let :G(I) G(J; M) G(I+J; M) be . The first purpose of this paper is to prove the following theorem.     

The quaternion group as a subgroup of the sphere braid groups

Let n 3. We prove that the quaternion group of order 8 is realisedas a subgroup of the sphere braid group B_n(²) if and only ifn is even. If n is divisible by 4, then the commutator subgroupof B_n(²) contains such a subgroup. Further, for all n 3, B_n(²)contains a subgroup isomorphic to the dicyclic group of order4n.     

Determination of Homomorphisms between Linear Groups of the same Degree over Division Rings

We determine all group homomorphisms SL_n(D) GL_n(E), where Dand E are division rings such that char D = char E and E isfinite-dimensional over its centre. Apart from the case wheren = 2 and D is F₂ or F_3, all non-trivial group homomorphismsSL_n(D) GL_n(E) are induced from homomorphisms and anti-homomorphismsD E.     

Coherent Products in a Finite Group along a Linear Ordering

Let C = (C, ) be a linear ordering, E a subset of {(x, y):x< y in C} whose transitive closure is the linear orderingC, and let :E G be a map from E to a finite group G = (G, •).We showed with M. Pouzet that, when C is countable, there isF E whose transitive closure is still C, and such that (p) = (x_o, x₁)•(x₁, x₂)•....•(x_{n– 1}, x_n) G depends only upon the extremities x₀, x_n ofp, where p = (x_o, x₁...,x_n) (with 1 n < ) is a finite sequencefor which (x_i, x_{i + 1}) F for all i < n. Here, we show thatthis property does not hold if C is the real line, but is stilltrue if C does not embed an ₁-dense linear ordering, or evena 2-dense linear ordering when Martin's Axiom holds (it followsin particular that it is independent of ZFC for linear orderingsof size ). On the other hand, we prove that this property isalways valid if E = {(x,y):x < y in C}, regardless of anyother condition on C.     

Cerf Theory for Graphs

We develop a deformation theory for k-parameter families ofpointed marked graphs with fixed fundamental group F_n. Applicationsinclude a simple geometric proof of stability of the rationalhomology of Aut(F_n), computations of the rational homology insmall dimensions, proofs that various natural complexes of freefactorizations of F_n are highly connected, and an improvementon the stability range for the integral homology of Aut(F_n).     

Binomial Sums and Functions of Exponential Type

Let (a_n)_n0 be a sequence of complex numbers, and, for n0, let A number of results are proved relating the growth of the sequences(b_n) and (c_n) to that of (a_n). For example, given p0, if b_n= O(n^p and for all > 0,then a_n=0 for all n > p. Also, given 0 < p < 1, then for all > 0 if and onlyif . It is further shown that, given rß > 1, if b_n,c_n=O(rßⁿ), then a_n=O(ⁿ),where , thereby proving a conjecture of Chalendar, Kellay and Ransford. The principal ingredientsof the proogs are a Phragmén-Lindelöf theorem forentire functions of exponential type zero, and an estimate forthe expected value of e^(X), where X is a Poisson random variable.2000 Mathematics Subject Classification 05A10 (primary), 30D15,46H05, 60E15 (secondary).     

BENFORD'S LAW FOR THE 3x + 1 FUNCTION

Benford's law (to base B) for an infinite sequence {x_k : k 1} of positive quantities x_k is the assertion that {log_B x_k: k 1} is uniformly distributed (mod 1). The 3x + 1 functionT(n) is given by T(n) = (3n + 1)/2 if n is odd, and T(n) = n/2if n is even. This paper studies the initial iterates x_k = T^(k)(x₀)for 1 k N of the 3x + 1 function, where N is fixed. It showsthat for most initial values x₀, such sequences approximatelysatisfy Benford's law, in the sense that the discrepancy ofthe finite sequence {log_B x_k : 1 k N} is small.     

The Density of Rational Points on Non-Singular Hypersurfaces, I

For any n 3, let F Z[X₀, ..., X_n] be a form of degree d 5that defines a non-singular hypersurface X Pⁿ. The main resultin this paper is a proof of the fact that the number N(F; B)of Q-rational points on X which have height at most B satisfies , for any > 0. The implied constantin this estimate depends at most upon d, and n. New estimatesare also obtained for the number of representations of a positiveinteger as the sum of three dth powers, and for the paucityof integer solutions to equal sums of like polynomials. 2000Mathematics Subject Classification 11G35 (primary), 11P05, 14G05(secondary).     

On the Optimum Criterion of Polynomial Stability

The purpose of this note is to answer the question raised byNie & Xie (1987). Let f(x)=a₀xⁿ+a₁x^n–1+...+a_n be apositive-coefficient polynomial. The numbers ₁=a_i-1a_i+2/a_ia_i+1(i=1, ..., n–2) are called determining coefficients. Theoptimum criterion problem was posed as follows: for n3, findthe maximal number (n) such that the polynomial f(x) is stableif _i < (n) (1in–2). For n6, we show that (n)=ß,where ß is the unique real root of the equation x(x+1)²=1.     

The Convergence of a Class of Quasimonotone Reaction-Diffusion Systems

It is proved that every solution of the Neumann initial-boundaryproblem converges to some equilibrium, if the system satisfies (i) F_i/u_j 0 for all 1 i j n, (ii) F(u * g(s)) h(s) * F(u) wheneveru and 0 s 1, where x *y = (x₁y₁, ..., x_ny_n) and g, h : [0, 1] [0, 1]ⁿ are continuousfunctions satisfying g_i(0) = h_i(0) = 0, g_i(1) = h_i(1) = 1, 0< g_i(s); h_i(s) < 1 for all s (0, 1) and i = 1, 2, ...,n, and (iii) the solution of the corresponding ordinary differentialequation system is bounded in . We also study the convergence of the solution of the Lotka–Volterrasystem where r_i > 0, 0, and a_ij 0 for i j.     

The Singular Homology of the Hawaiian Earring

The singular homology groups of compact CW-complexes are finitelygenerated, but the groups of compact metric spaces in generalare very easy to generate infinitely and our understanding ofthese groups is far from complete even for the following compactsubset of the plane, called the Hawaiian earring: Griffiths [11] gave a presentation of the fundamental groupof H and the proof was completed by Morgan and Morrison [15].The same group is presented as the free -product of integers Z in [4, Appendix]. Hence the firstintegral singular homology group H₁(H) is the abelianizationof the group . These results have been generalized to non-metrizable counterparts H_I of H(see Section 3). In Section 2 we prove that H₁(X) is torsion-free and H_i(X) =0 for each one-dimensional normal space X and for each i 2.The result for i 2 is a slight generalization of [2, Theorem5]. In Section 3 we provide an explicit presentation of H₁(H)and also H₁(H_I) by using results of [4]. Throughout this paper, a continuum means a compact connectedmetric space and all maps are assumed to be continuous. Allhomology groups have the integers Z as the coefficients. Thebouquet with n circles is denoted by B_n. The base point (0, 0) of B_n is denoted by o forsimplicity.     

The Analogue of Buchi's Problem for Rational Functions

Büchi's problem asked whether there exists an integer Msuch that the surface defined by a system of equations of theform has no integer pointsother than those that satisfy ±x_n = ± x₀ + n (the± signs are independent). If answered positively, itwould imply that there is no algorithm which decides, givenan arbitrary system Q = (q1,...,q_r) of integral quadratic formsand an arbitrary r-tuple B = (b₁,...,b_r) of integers, whetherQ represents B (see T. Pheidas and X. Vidaux, Fund. Math. 185(2005) 171–194). Thus it would imply the following strengtheningof the negative answer to Hilbert's tenth problem: the positive-existentialtheory of the rational integers in the language of additionand a predicate for the property ‘x is a square’would be undecidable. Despite some progress, including a conditionalpositive answer (depending on conjectures of Lang), Büchi'sproblem remains open. In this paper we prove the following: (A) an analogue of Büchi's problem in rings of polynomialsof characteristic either 0 or p 17 and for fields of rationalfunctions of characteristic 0; and (B) an analogue of Büchi's problem in fields of rationalfunctions of characteristic p 19, but only for sequences thatsatisfy a certain additional hypothesis. As a consequence we prove the following result in logic. Let F be a field of characteristic either 0 or at least 17 andlet t be a variable. Let L_t be the first order language whichcontains symbols for 0 and 1, a symbol for addition, a symbolfor the property ‘x is a square’ and symbols formultiplication by each element of the image of [t] in F[t].Let R be a subring of F(t), containing the natural image of[t] in F(t). Assume that one of the following is true: (i) R F[t]; (ii) the characteristic of F is either 0 or p 19. Then multiplication is positive-existentially definable overthe ring R, in the language L_t. Hence the positive-existentialtheory of R in L_t is decidable if and only if the positive-existentialring-theory of R in the language of rings, augmented by a constant-symbolfor t, is decidable.     

All Groups are Outer Automorphism Groups of Simple Groups

It is shown that each group is the outer automorphism groupof a simple group. Surprisingly, the proof is mainly based onthe theory of ordered or relational structures and their symmetrygroups. By a recent result of Droste and Shelah, any group isthe outer automorphism group Out (Aut T) of the automorphismgroup Aut T of a doubly homogeneous chain (T, ). However, AutT is never simple. Following recent investigations on automorphismgroups of circles, it is possible to turn (T, ) into a circleC such that Out (Aut T) Out (Aut C). The unavoidable normalsubgroups in Aut T evaporate in Aut C, which is now simple,and the result follows.     

Arithmetic properties of Apery numbers

Let (A_n)_n1 be the sequence of Apéry numbers with a generalterm given by . In thispaper, we prove that both the inequalities (A_n) > c₀ loglog log n and P(A_n) > c₀ (log n log log n)^1/2 hold fora set of positive integers n of asymptotic density 1. Here,(m) is the number of distinct prime factors of m, P(m) is thelargest prime factor of m and c₀ > 0 is an absolute constant.The method applies to more general sequences satisfying botha linear recurrence of order 2 with polynomial coefficientsand certain Lucas-type congruences.