a cauchy problem for a nonlinear convection-diffusion equation with periodic rapidly oscillating coefficients is studied. under the assumption that the convection term is large, it is proved that the limit (homogenized) equation is a nonlinear diffusion equation which shows dispersion effects. the convergence of the homogenization procedure is justified by using a new version of a two-scale convergence technique adapted to rapidly moving coordinates.

The Cauchy problem is studied for the nonlinear equations with fractional power of the negative Laplacian \[ \left\{ \begin{array}{@{}r@{\,}c@{\,}l@{\qquad}l} u_{t}+( -\Delta ) ^{{\alpha}/{2}}u+u^{1+\sigma } &=&0, & x\in {\mathbf{R}}^{n},\text{ }t>0, \\[4pt] u( 0,x) &=& u_{0} ( x), &x\in {\mathbf{R}}^{n}, \end{array} \right. \label{A} \] where $\alpha \in ( 0,2)$, with critical $\sigma ={\alpha }/{ n}$ and sub-critical $\sigma \in ( 0,{\alpha }/{n}) $ powers of the nonlinearity. Let $u_{0}\,{\in}\, \mathbf{L}^{1,a}\cap \mathbf{L}^{\infty }\cap \mathbf{C}, $$u_{0}( x) \,{\geq}\, 0$ in $\mathbf{R}^{n},$$\theta \,{=}\,\int_{ \mathbf{R}^{n}}u_{0}( x) \,dx\,{>}\,0.$ The case of not small initial data is of interest. It is proved that the Cauchy problem has a unique global solution $u \in \mathbf{C}( [0,\infty); \mathbf{L}^{\infty }\cap \mathbf{L}^{1,a}\cap \mathbf{C})$ and the large time asymptotics are obtained.

We present a method for computing the 3-point genus zero Gromov–Witten invariants of the complex flag manifold $G/B$ from the relations of the small quantum cohomology algebra $QH^\ast G/B$ ($G$ is a complex semisimple Lie group and $B$ is a Borel subgroup). In [3] and [9], at least in the case $G={\rm GL}_n{\bf C}$, two algebraic/combinatoric methods have been proposed, based on suitably designed axioms. Our method is quite different, being differential geometric in nature; it is based on the approach to quantum cohomology described in [7], which is in turn based on the integrable systems point of view of Dubrovin and Givental.

an approximation theorem in inhomogeneous orlicz–sobolev spaces is proved which allows a second-order parabolic equation in orlicz spaces to be solved. a trace result is also given which shows that the solutions are continuous with respect to time.

the paper considers the eigenvalue problem \[ -\delta u-\alpha u+\lambda g(x)u=0\quad \mbox{with}u\in h^{1}(\mathbb{r}^{n}),\;u\neq0, \] where $\alpha,\lambda\in\mathbb{r}$ and \[g(x)\equiv0\mbox{on}\overline{\omega},\quad\ g(x)\in(0,1]\mbox{on}{\mathbb{r}^{n}}\setminus{\overline{\omega}}\quad\mbox{and}\quad\lim_{|x|\rightarrow+\infty}g(x)=1 \] for some bounded open set $\omega\in\mathbb{r}^{n}$.

given $\alpha>0$, does there exist a value of $\lambda>0$ for which the problem has a positive solution? it is shown that this occurs if and only if $\alpha$ lies in a certain interval $(\gamma,\xi_{1})$ and that in this case the value of $\lambda$ is unique, $\lambda=\lambda(\alpha)$. the properties of the function $\lambda(\alpha)$ are also discussed.

For $(\l,a)\in \matbb{C}^*\times \mathbb{C}$, let $f_{\lambda,a}$ be the rational map defined by $f_{\lambda,a}(z) \,{=}\, \lambda z^2 {(az+1)/(z+a)}$. If $\alpha\in \mathbb{R}/\mathbb{Z}$ is a Brjuno number, we let ${\cal D}_\alpha$ be the set of parameters $(\lambda,a)$ such that $f_{\lambda,a}$ has a fixed Herman ring with rotation number $\alpha$ (we consider that $({\it e}^{2i\pi\alpha}{,}0)\,{\in}\, {\cal D}_\alpha$). Results obtained by McMullen and Sullivan imply that, for any $g\in {\cal D}_\alpha$, the connected component of ${\cal D}_\alpha\cap (\mathbb{C}^*\times(\mathbb{C}\setminus \{0,1\}))$ that contains g is isomorphic to a punctured disk.

We show that there is a holomorphic injection $\cal{F}_\alpha\,{:}\,\mathbb{D}\,{\longrightarrow}\, {\cal D}_\alpha$ such that $\cal{F}_\alpha(0) = ({\it e}^{2i\pi \alpha},0)$ and $\cal{F}_\alpha'(0)=(0,r_\alpha),$ where $r_\alpha$ is the conformal radius at 0 of the Siegel disk of the quadratic polynomial $z\longmapsto {\it e}^{2i\pi \alpha}z(1+z)$.

As a consequence, we show that for $a\in (0,1/3)$, if $f_{\l,a}$ has a fixed Herman ring with rotation number $\alpha$ and if $m_a$ is the modulus of the Herman ring, then, as $a\,{\rightarrow}\,0$, we have ${\it e}^{\pi m_a} \,{=} ({r_\alpha}/{a}) + {\cal O}(a).$

We finally explain how to adapt the results to the complex standard family $z\,{\longmapsto} \lambda z {\it e}^{({a}/{2})(z-1/z)}$.

The Kronecker quiver $K$ is considered, and the relations for the specialisation at $q=0$ of the generic composition algebra are given, as well as those for Reineke's composition monoid. As a corollary, it is deduced that the composition monoid is a proper factor of the specialisation of the composition algebra. A normal form is also obtained for the varieties occurring in the composition monoid in terms of Schur roots.

A Blaschke product $B$ with zero-sequence $(a_n)$ is called almost interpolating if the inequality $\liminf_n(1-|a_n|^2)|B'(a_n)|\geqslant \d>0$ holds. The sets $U$ for which there exists a Blaschke product $B$ such that $(a-B)/(1-\ov a B)$ is almost interpolating if and only if $a \in U$ are studied. Examples of such sets include open sets, containing the origin, and whose complement is the closure of an arbitrary set of concentric open arcs around the origin or open sets whose complement is of zero logarithmic capacity. Results on the range of interpolating Blaschke product s on the set of trivial points in the spectrum of $H^{\infty}$ are deduced.

the analytic concepts of martingale type $p$ and cotype $q$ of a banach space have an intimate relation with the geometric concepts of $p$-concavity and $q$-convexity of the space under consideration, as shown by pisier. in particular, for a banach space $x$, having martingale type $p$ for some $p{>}1$ implies that $x$ has martingale cotype $q$ for some $q{<}\infty$.

the generalisation of these concepts to linear operators was studied by the author, and it turns out that the duality above only holds in a weaker form. an example is constructed showing that this duality result is best possible.

so-called random martingale unconditionality estimates, introduced by garling as a decoupling of the unconditional martingale differences (umd) inequality, are also examined.

it is shown that the random martingale unconditionality constant of $l_\infty^{2^n}$ for martingales of length $n$ asymptotically behaves like $n$. this improves previous estimates by geiss, who needed martingales of length $2^n$ to show this asymptotic. at the same time the order in the paper is the best that can be expected.

We extend uncertainty principles which are valid for the Fourier transform to the setting of the ambiguity function. A general result is established for annihilating sets: strongly/weakly annihilating sets for the Fourier transform yield such sets for the ambiguity function, extending a result known for sets of finite measure. We apply this to sublevel sets of nondegenerate quadratic forms. Our main result is a sharp version of Beurling's uncertainty principle for the ambiguity function.

the interplay between the behaviour of approximately convex (and approximately affine) functions on the unit ball of a banach space and the geometry of banach $k$-spaces is studied.

The paper examines a higher-order ordinary differential equation of the form $\mathcal{P}[u]\,{:=} \sum_{j,k=0}^{m}D^{j}a_{jk}D^{k}u \,{=}\, \lambda{u}, x\in[0,b)$, where $D\,{=}\,i({d}/{dx})$, and where the coefficients $a_{jk}$, $j,k\in[0,m]$, with $a_{mm}\,{=}\,1$, satisfy certain regularity conditions and are chosen so that the matrix $(a_{jk})$ is hermitean. It is also assumed that $m\,{>}\,1$. More precisely, it is proved, using Paley–Wiener methods, that the corresponding spectral measure determines the equation up to conjugation by a function of modulus 1. The paper also discusses under which additional conditions the spectral measure uniquely determines the coefficients $a_{jk}$, $j,k\in[0,m]$, $j+k\neq{2m}$, as well as $b$ and the boundary conditions at 0 and at $b$ (if any).

The natural geometric setting of quadrics commuting with a Hermitian surface of ${\rm PG}(3,q^2)$, q odd, is adopted and a hemisystem on the Hermitian surface ${\cal H}(3,q^2)$ admitting the group $\mathrm{{P}}\Omega^-(4,q)$ is constructed, yielding a partial quadrangle ${\rm PQ}((q-1)/2,q^2,(q-1)^2/2)$ and a strongly regular graph srg$((q^3+1)(q+1)/2,(q^2+1)(q-1)/2,(q-3)/2,(q-1)^2/2)$. For $q>3$, no partial quadrangle or strongly regular graph with these parameters was previously known, whereas when $q=3$, this is the Gewirtz graph. Thas conjectured that there are no hemisystems on ${\cal H}(3,q^2)$ for $q>3$, so these are counterexamples to his conjecture. Furthermore, a hemisystem on ${\cal H}(3,25)$ admitting $3.A_7.2$ is constructed. Finally, special sets (after Shult) and ovoids on ${\cal H}(3,q^2)$ are investigated.

Schottky uniformizations are provided of every closed Riemann surface $S$ of genus $g \in \{3,4\}$ admitting the symmetric group ${\mathcal S}_{4}$ as group of conformal automorphisms. These Schottky uniformizations reflect the group ${\mathcal S}_{4}$ and permit concrete representations of ${\mathcal S}_{4}$ to be obtained in the respective symplectic group $\mbox{Sp}_{g}({\mathbb Z})$. Their corresponding fixed points, in the Siegel space, give principally polarized Abelian varieties of dimension $g$. For $g=3$ and for some cases of $g=4$ they turn out to be holomorphically equivalent to the product of elliptic curves.

We generalise group algebras to other algebraic objects with bounded Hilbert space representation theory; the generalised group algebras are called ‘host’ algebras. The main property of a host algebra is that its representation theory should be isomorphic (in the sense of the Gelfand–Raikov theorem) to a specified subset of representations of the algebraic object. Here we obtain both existence and uniqueness theorems for host algebras as well as general structure theorems for host algebras. Abstractly, this solves the question of when a set of Hilbert space representations is isomorphic to the representation theory of a C*-algebra. To make contact with harmonic analysis, we consider general convolution algebras associated to representation sets, and consider conditions for a convolution algebra to be a host algebra.

global weierstrass representations are derived for complete minimal surfaces obtained by substituting the planar end of the costa surface by scherk ends. this gives rise to singly periodic examples, of which the fundamental piece is proved to be embedded by a simple geometric technique. this technique can be generalised for embeddedness proof of future examples of the same nature.

To gain understanding of the deformations of determinants and Pfaffians resulting from deformations of matrices, the deformation theory of composites $f\,{\circ}\,F$ with isolated singularities is studied, where $f\,{:}\,Y\,{\longrightarrow}\,\mathbb{C}$ is a function with (possibly non-isolated) singularity and $F:X\,{\longrightarrow}\, Y$ is a map into the domain of $f$, and $F$ only is deformed. The corresponding $T^1(F)$ is identified as (something like) the cohomology of a derived functor, and a canonical long exact sequence is constructed from which it follows that \[\tau=\mu(f\circ F)-\beta_0+\beta_1\], where $\tau$ is the length of $T^1(F)$ and $\beta_i$ is the length of $\mbox{Tor}_i^{\cal{O}_Y}({\cal O}_Y{/}J_f,\cal{O}_X)$. This explains numerical coincidences observed in lists of simple matrix singularities due to Bruce, Tari, Goryunov, Zakalyukin and Haslinger. When $f$ has Cohen–Macaulay singular locus (for example when $f$ is the determinant function), relations between $\tau$ and the rank of the vanishing homology of the zero locus of $f\circ F$ are obtained.

the total space of the tangent bundle of a kähler manifold admits a canonical kähler structure. parallel translation identifies the space ${\mathbb{t}}$ of oriented affine lines in ${\mathbb{r}}^3$ with the tangent bundle of $s^2$. thus the round metric on $s^2$ induces a kähler structure on ${\mathbb{t}}$ which turns out to have a metric of neutral signature. it is shown that the identity component of the isometry group of this metric is isomorphic to the identity component of the isometry group of the euclidean metric on ${\mathbb{r}}^3$.

the geodesics of this metric are either planes or helicoids in ${\mathbb{r}}^3$. the signature of the metric induced on a surface $\sigma$ in ${\mathbb{t}}$ is determined by the degree of twisting of the associated line congruence in ${\mathbb{r}}^3$, and it is shown that, for $\sigma$ lagrangian, the metric is either lorentz or totally null. for such surfaces it is proved that the keller–maslov index counts the number of isolated complex points of ${\mathbb{j}}$ inside a closed curve on $\sigma$.

Given a directed graph, there exist a universal operator algebra and universal $C^*$-algebra associated to the directed graph. In this paper we give intrinsic constructions for these objects. We also provide an explicit construction for the maximal $C^*$-algebra of an operator algebra. We discuss uniqueness of the universal algebras for finite graphs, showing that for finite graphs the graph is an isomorphism invariant for the universal operator algebra of a directed graph. We show that the underlying undirected graph is a Banach algebra isomorphism invariant for the universal $C^*$-algebra of a directed graph.

Consider the image of the Monge–Ampère operator acting on bounded functions, defined on a compact Kähler manifold, whose sum with the local Kähler potential is plurisubharmonic. It is shown that a nonnegative Borel measure belongs to this image if and only if it belongs to the image locally. In particular, those measures form a convex set.