The paper defines a new type of product of association schemes (and of the related objects, permutation groups and orthogonal block structures), which generalizes the direct and wreath products (which are referred to as ‘crossing’ and ‘nesting’ in the statistical literature). Given two association schemes $\mathcal{Q}_r$ for $r\,{=}\,1,2$, each having an inherent partition $F_r$ (that is, a partition whose equivalence relation is a union of adjacency relations in the association scheme), a product of the two schemes is defined, which reduces to the direct product if $F_1=U_1$ or $F_2=E_2$, and to the wreath product if $F_1 = E_1$ and $F_2 = U_2$, where $E_r$ and $U_r$ are the relation of equality and the universal relation on $\mathcal{Q}_r$. The character table of the crested product is calculated, and it is shown that, if the two schemes $\mathcal{Q}_1$ and $\mathcal{Q}_2$ have formal duals, then so does their crested product (and a simple description of this dual is given). An analogous definition for permutation groups with intransitive normal subgroups is created, and it is shown that the constructions for association schemes and permutation groups are related in a natural way.

The definition can be generalized to association schemes with families of inherent partitions, or permutation groups with families of intransitive normal subgroups. This time the correspondence is not so straightforward, and it works as expected only if the inherent partitions (or orbit partitions) form a distributive lattice.

The paper concludes with some open problems.

the set of squares $n^2$, $n<2^k$, is considered and the sum of binary digits $s(n^2)$ is split up into two parts $s_{[<k]}(n^2)+s_{[\ge k]}(n^2)$, where $s_{[<k]}(n^2) = s(n^2{\rm mod}2^k)$ collects the first $k$ digits and $s_{[\ge k]}(n^2) = s(\lfloor n^2/2^k\rfloor)$ collects the remaining digits. very precise results on the distribution of $s_{[<k]}(n^2)$ and $s_{[\ge k]}(n^2)$ are presented. for example, asymptotic formulae are provided for the numbers $\#\{n< 2^k{:} s_{[<k]}(n^2) = m\}$ and $\#\{n< 2^k {:} s_{[\ge k]}(n^2) = m\}$ and it is shown that these partial sum-of-digits functions are asymptotically equidistributed in residue classes. these results are prompted by a conjecture by gelfond saying that the (total) sum-of-digits function $s(n^2)$ is asymptotically equidistributed in residue classes.

We study genus 5 curves C with a fixed point free involution. We give a geometrical (embedded) characterisation of these curves among all genus 5 curves: the points of the Prym variety associated to the involution give embeddings of the curve C in ${{\mathbb P}}_3$ so that C has infinitely many quadrisecant lines. Conversely, any genus 5 curve having such an embedding is endowed with a fixed point free involution and the embedding is given by a point of the Prym variety.

It is proved that all correlations of the sequence of Farey fractions exist and explicit formulas are provided for the correlation measures.

the convergence case of a khintchine-type theorem for a large class of hyperplanes is obtained. the approach to the problem is from a dynamical viewpoint, and a method due to kleinbock and margulis is modified to prove the result.

The work in this paper is part of an ongoing program to classify maximal orders on surfaces via their ramification data. Del Pezzo orders and ruled orders have already been classified by the authors and others. In this paper, we classify numerically Calabi–Yau orders which are the noncommutative analogues of minimal surfaces of Kodaira dimension zero.

Standard codeterminants for Donkin's symplectic Schur algebra $\bar{S}(n,r)$ are defined. It is shown that they form a basis of $\bar{S}(n,r)$. They are then used to give a purely combinatorial proof that $\bar{S}(n,r)$ is quasi-hereditary.

it is conjectured that a rational map whose coefficients are algebraic over $\mathbb q_p$ has no wandering components of the fatou set. benedetto has shown that any counterexample to this conjecture must have a wild recurrent critical point. we provide the first examples of rational maps whose coefficients are algebraic over $\mathbb q_p$ and that have a (wild) recurrent critical point. in fact, it is shown that there is such a rational map in every one-parameter family of rational maps that is defined over a finite extension of $\mathbb q_p$ and that has a misiurewicz bifurcation.

Explicit bounds are given for the residues at $s\,{=}\,1$ of the Dedekind zeta functions of number fields. As a consequence, a simple proof of the Brauer-Siegel theorem and explicit lower bounds for class numbers of number fields are obtained. Compared with Stark's original approach, the paper is explicit and more satisfactory for number fields containing quadratic subfields. Examples are given of fully explicit lower bounds for class numbers of various types of number fields, for example normal and non-normal number fields of odd degree, with an emphasis on cubic fields, real cyclic quartic number fields, and non-normal quartic number fields containing an imaginary quadratic subfield.

Finitely generated modules with finite $F$-representation type over Noetherian (local) rings of prime characteristic $p$ are studied. If a ring $R$ has finite $F$-representation type or, more generally, if a faithful $R$-module has finite $F$-representation type, then tight closure commutes with localizations over $R$. $F$-contributors are also defined, and they are used as an effective way of characterizing tight closure. Then it is shown that $\lim_{e \to \infty} ({\#(\e M,M_i)}/ {(ap^d)^e})$ always exists under the assumption that $(R,\m)$ satisfies the Krull–Schmidt condition and $M$ has finite $F$-representation type by $\{ M_1, M_2, \dots, M_s \}$, in which all the $M_i$ are indecomposable $R$-modules that belong to distinct isomorphism classes and $a=[R/\m:(R/\m)^p]$.

The paper investigates the partial regularity of the minimizers for quadratic functionals whose integrands have VMO coefficients in principal part and nonlinear terms that are Carathéodory functions. We use some majorizations for the functional, rather than the well known Euler equation associated to it.

three families of pairs of curves are presented; each pair consists of geometrically non-isomorphic curves whose jacobians are isomorphic to one another as unpolarized abelian varieties. each family is parametrized by an open subset of ${\mathbb p}^1$. the first family consists of pairs of genus-2 curves whose equations are given by simple expressions in the parameter; the curves in this family have reducible jacobians. the second family also consists of pairs of genus-2 curves, but generically the curves in this family have absolutely simple jacobians. the third family consists of pairs of genus-3 curves, one member of each pair being a hyperelliptic curve and the other a plane quartic. examples from these families show that in general it is impossible to tell from the jacobian of a genus-2 curve over $\mathbb q$ whether or not the curve has rational points – or indeed whether or not it has real points. the families are constructed using methods that depend on earlier joint work with franck leprévost and bjorn poonen, and on peter bending's explicit description of the curves of genus 2 whose jacobians have real multiplication by $\mathbb z[\sqrt{2}]$.

a new viewpoint is offered on some of the generic structures constructed using hrushovski's predimensions and it is shown that they are natural reducts of quite straightforward trivial, one-based stable structures.

The spectral gaps and thus the exponential rates of convergence to equilibrium are compared for ergodic one-dimensional diffusions on an interval. One of the results may be thought of as the diffusion analogue of a recent result for the spectral gap of one-dimensional Schrödinger operators. The similarities and differences between spectral gap results for diffusions and for Schrödinger operators are also discussed.

The defining ideal $I_{\bb X}$ of a set of points $\bb{X}$ in $\bb{P}^n_1 x \ldots x \bb{P}n_k$ is investigated with a special emphasis on the case when $\bb{X}$ is in generic position, that is, $\bb{X}$ has the maximal Hilbert function. When $\bb{X}$ is in generic position, the degrees of the generators of the associated ideal $I_{\bb X}$ are determined. $\nu(I_{\bb X})$ denotes the minimal number of generators of $I_{\bb X}$, and this description of the degrees is used to construct a function $v(s;n_1,\ldots,n_k)$ with the property that $\nu(I_{\bb X})\,{\geq}\, v(s;n_1,\ldots,n_k)$ always holds for $s$ points in generic position in $\bb{P}n_1 x \ldots x \bb{P}^n_k$. When $k\,{=}\,1$, $v(s;n_1)$ equals the expected value for $\nu(I_{\bb X})$ as predicted by the ideal generation conjecture. If $k\,{\geq}\, 2$, it is shown that there are cases with $\nu(I_{\bb X}) > v(s;n_1,\ldots,n_k$). However, computational evidence suggests that in many cases $\nu(I_{\bb X})\,{=}\, v(s;n_1,\ldots,n_k$).

Homogeneous fragmentations describe the evolution of a unit mass that breaks down randomly into pieces as time passes. They can be thought of as continuous time analogues of a certain type of branching random walk, which suggests the use of time-discretization to shift known results from the theory of branching random walks to the fragmentation setting. In particular, this yields interesting information about the asymptotic behaviour of fragmentations.

On the other hand, homogeneous fragmentations can also be investigated using a powerful technique of discretization of space due to Kingman, namely, the theory of exchangeable partitions of $\mathbb{N}$. Spatial discretization is especially well suited to the direct development for continuous times of the conceptual method of probability tilting of Lyons, Pemantle and Peres.

let $l$ denote a right-invariant sub-laplacian on an exponential, hence solvable lie group $g$, endowed with a left-invariant haar measure. depending on the structure of $g$, and possibly also that of $l, l$ may admit differentiable $l^p$-functional calculi, or may be of holomorphic $l^p$-type for a given $p{\ne} 2$. ‘holomorphic $l^p$-type’ means that every $l^p$-spectral multiplier for $l$ is necessarily holomorphic in a complex neighbourhood of some non-isolated point of the $l^2$-spectrum of $l$. this can in fact only arise if the group algebra $l^1(g)$ is non-symmetric.

assume that $p{\ne} 2$. for a point $\ell$ in the dual $\frak{g}^*$ of the lie algebra $\frak{g}$ of $g$, denote by $\omega(\ell){=}\ad^*(g)\ell$ the corresponding coadjoint orbit. it is proved that every sub-laplacian on $g$ is of holomorphic $l^p$-type, provided that there exists a point $\ell{\in} \frak{g}^*$ satisfying boidol's condition (which is equivalent to the non-symmetry of $l^1(g)$), such that the restriction of $\omega(\ell)$ to the nilradical of $\frak{g}$ is closed. this work improves on results in previous work by christ and müller and ludwig and müller in twofold ways: on the one hand, no restriction is imposed on the structure of the exponential group $g$, and on the other hand, for the case $p{>}1$, the conditions need to hold for a single coadjoint orbit only, and not for an open set of orbits.

it seems likely that the condition that the restriction of $\omega(\ell)$ to the nilradical of $\frak{g}$ is closed could be replaced by the weaker condition that the orbit $\omega(\ell)$ itself is closed. this would then prove one implication of a conjecture by ludwig and müller, according to which there exists a sub-laplacian of holomorphic $l^1$ (or, more generally, $l^p$) type on $g$ if and only if there exists a point $\ell{\in} \frak{g}^*$ whose orbit is closed and which satisfies boidol's condition.

By using Morse theory, we study the existence and multiplicity of nontrivial solutions for a class of p-sublinear p-Laplacian elliptic equations.

It is proved that the associated variety of a Poisson prime ideal of the centre of a symplectic reflection algebra at parameter $t \,{=}\, 0$ is irreducible.

On the basis of some new Liouville theorems, under suitable conditions, a priori estimates are obtained of positive solutions of the problem \[-\Delta _pu=\lambda u^{\alpha }-a(x)u^q\quad \mbox{in}\;\Omega,\qquad u|_{\partial \Omega }=0,\] where $\Omega \subset {\mathbb{R}}^N$ ($N\geq 2$) is a bounded smooth domain, $p>1$ and λ is a parameter, α, q are given constants such that $p-1<\alpha <p^*-1$, $\alpha <q$, $p^*=Np/(N-p)$ if $N > p$ and $p^*=\infty $ when $N\leq p$, and $a(x)$ is a continuous nonnegative function. Making use of the Leray–Schauder degree of a compact mapping and a priori estimates, the paper finds that the problem above possesses at least one positive solution. It also discusses the corresponding perturbed problem, where $a(x)$ is replaced by $a(x)+\epsilon$, $\epsilon>0$. The results are strikingly different from those obtained for the case $\alpha=p-1$.