Assume that $G$ is a group covered by countably many sets $X_n,\,n\,{<}\,\omega$. It is proved that $G$ is generated in two or three steps by a small number of the sets $X_n$. These results are generalized for countable coverings of types and countable colourings of graphs.

The conjecture stated in an earlier paper by the author that there is a constant $\lambda$ (independent from both $n$ and $k$) such that $S(K_{n}^{d})\ge \lambda n^{d-1}$ holds for every $n\ge 2$ and $d\ge 2$, where $S(K_{n}^{d})$ is the length of the longest snake (cycle without chords) in the Cartesian product $K_{n}^{d}$ of $d$ copies of the complete graph $K_{n}$, is proved.

It is proved that there does not exist a set of four positive integers with the property that the product of any two of its distinct elements plus their sum is a perfect square. This settles an old problem investigated by Diophantus and Euler.

Let $K$ be a function field of genus $g$ with a finite constant field ${\mathbb{F}}_q$. Choose a place $\infty$ of $K$ of degree $\delta$ and let ${\mathbb{C}}$ be the arithmetic Dedekind domain consisting of all elements of $K$ that are integral outside $\infty$. An explicit formula is given (in terms of $q$, $g$ and $\delta$) for the minimum index of a non-congruence subgroup in SL$_2({\mathcal{C}})$. It turns out that this index is always equal to the minimum index of an arbitrary proper subgroup in SL$_2({\mathcal{C}})$. The minimum index of a normal non-congruence subgroup is also determined.

Let $\textbf{F}\,{=}\,(F_1, \ldots, F_m)$ be an $m$-tuple of primitive positive binary quadratic forms and let $U_{\textbf{F}}(x)$ be the number of integers not exceeding $x$ that can be represented simultaneously by all the forms $F_j$, $j = 1, \ldots, m$. Sharp upper and lower bounds for $U_{\textbf{F}}(x)$ are given uniformly in the discriminants of the quadratic forms.

As an application, a problem of Erdős is considered. Let $V(x)$ be the number of integers not exceeding $x$ that are representable as a sum of two squareful numbers. Then $V(x) = x(\log x)^{-\alpha+o(1)}$ with $\alpha\,{=}\,1-2^{-1/3}\,{=}\,0.206\ldots$.

Several important cases of vector bundles with extra structure (such as Higgs bundles and triples) may be regarded as examples of twisted representations of a finite quiver in the category of sheaves of modules on a variety/manifold/ringed space. It is shown that the category of such representations is an abelian category with enough injectives by the construction of an explicit injective resolution. Using this explicit resolution, a long exact sequence is found that computes the Ext groups in this new category in terms of the Ext groups in the old category. The quiver formulation is directly reflected in the form of the long exact sequence. It is also shown that under suitable circumstances, the Ext groups are isomorphic to certain hypercohomology groups.

Several results about the multiplicities of indecomposable injectives in the minimal injective resolution of a ring exist in the literature. Mostly these apply to universal enveloping algebras of finite dimensional solvable Lie algebras, and to Gorenstein noetherian PI local rings. These results are unified and extended to the much wider class of rings with Auslander dualizing complexes.

It is shown that, for finitely generated residually soluble groups, a condition weaker than polynomial growth guarantees virtual nilpotence. Let $G$ be a residually soluble group having a finite generating set $X$, and suppose that the number $\ga_X(n)$ of elements of $G$ that are products of at most $n$ elements of $X\cup X^{-1}$ satisfies $\ga_X(n)\leqslant e^{\alpha(n)}$ for each $n$, where $\alpha(n)/e^{(1/2)(\ln n)^{1/2}}\to\infty$ as $n\to\infty$; then $G$ is virtually nilpotent.

Let $C$ be a genus 2 algebraic curve defined by an equation of the form $y^2\,{=}\,x(x^2\,{-}\,1)(x\,{-} a)(x\,{-}\,1/a)$. As is well known, the five accessory parameters for such an equation can all be expressed in terms of $a$ and the accessory parameter $\frak b$ corresponding to $a$. The main result of the paper is that if $a'\,{=}\,\sqrt{1\,{-}\,{a}^2}$, which in general yields a non-isomorphic curve $C'$, then $\acb'a'({a'}^2\,{-}\,1) {=}\,{-}\frac38\,{-}\,\acb\,a\,({a}^2\,{-}\,1)$.

This is proven by it being shown how the uniformizing function from the unit disk to $C'$ can be explicitly described in terms of the uniformizing function for $C$.

Using critical point theory, some sufficient conditions are obtained for the existence of nonconstant $T$-periodic solutions of a class of second order self-adjoint difference equations.

Let $\tau$ be a conjugation, alias a conjugate linear isometry of order 2, on a complex Banach space $X$ and let $X^{\tau}$ be the real form of $X$ of $\tau$-fixed points. In contrast to the Dunford–Pettis property, the alternative Dunford–Pettis property need not lift from $X^{\tau}$ to $X$. If $X$ is a C*-algebra it is shown that $X^{\tau}$ has the alternative Dunford–Pettis property if and only if $X$ does and an analogous result is shown when $X$ is the dual space of a C*-algebra. One consequence is that both Dunford–Pettis properties coincide on all real forms of C*-algebras.

Criteria are found which classify Schatten class membership for operators on the Hardy space in terms of their action on the reproducing kernels and their derivatives. The Hilbert–Schmidt conditions are shown to be both necessary and sufficient for arbitrary operators. For other Schatten classes, the criteria are shown to be either necessary or sufficient for arbitrary operators, depending upon the exponent of the class. However, using known results of Peller and Luecking, the criteria are shown to be both necessary and sufficient for Hankel operators and Carleson embeddings.

Multidirectional mean value inequalities provide estimates of the difference of the extremal value of a function on a given bounded set and its value at a given point in terms of its (sub)-gradient at some intermediate point. A generalization of such multidirectional mean value inequalities is derived by using new infinitesimal conditions for a weak r-growth of the lower semicontinuous function along approximate trajectories of differential inclusions. This new form of the multidirectional mean value inequality does not rely on the linear structure of the underlying space and removes a traditional assumption of lower boundedness on the function.

The covariogram of a convex body $K$ provides the volumes of the intersections of $K$ with all its possible translates. Matheron conjectured in 1986 that this information determines $K$ among all convex bodies, up to certain known ambiguities. It is proved that this is the case if $K\subset{\mathbb R}^2$ is not ${\rm C}^1$, or it is not strictly convex, or its boundary contains two arbitrarily small ${\rm C}^2$ open portions ‘on opposite sides’. Examples are also constructed that show that this conjecture is false in ${\mathbb R}^n$ for any $n\geq 4$.

The Hessian measures of a (semi-)convex function can be introduced as coefficients of a local Steiner formula. The investigation of Hessian measures is continued by the provision of a geometric characterization of the support of these measures. Then the Radon–Nikodym derivative and the absolute continuity of Hessian measures with respect to Lebesgue measure are explored. As special cases of the results, known results for surface area measures of convex bodies are recovered.

Certain discrete-time Markov processes on locally compact metric spaces which arise from graph-directed constructions of fractal sets with place-dependent probabilities are studied. Such systems naturally extend finite Markov chains and inherit some of their properties. It is shown that the Markov operator defined by such a system has a unique invariant probability measure in the irreducible case and an attractive probability measure in the aperiodic case if the vertex sets form an open partition of the state space, the restrictions of the probability functions on their vertex sets are Dini-continuous and bounded away from zero, and the system satisfies a condition of contractiveness on average.

Many new universal relations are obtained between the Euler numbers of manifolds of singular supporting hyperplanes of an arbitrary generic smooth closed $k$-dimensional submanifold in ${{\mathbb R}}^n$ where $n\leq 7$ or $k=1$. These relations are applied to Barner-convex curves in an odd-dimensional space ${{\mathbb R}}^n$. A universal (nontrivial) linear relation is established between the numbers of singular supporting hyperplanes of various types but of the same total multiplicity $n$ of tangency with a given generic smooth closed connected Barner-convex curve in ${{\mathbb R}}^n$. The coefficients of this relation are defined by Catalan numbers.