We point out relations between Siciak’s homogeneous extremal function ${\Psi }_B$ and the Cauchy-Poisson transform in case $B$ is a ball in ℝ². In particular, we find effective formulas for ${\Psi }_B$ for an important class of balls. These formulas imply that, in general, ${\Psi }_B$ is not a norm in ℂ².

We recall first Mather's Lemma providing effective necessary and sufficient conditions for a connected submanifold to be contained in an orbit. We show that two homogeneous polynomials having isomorphic Milnor algebras are right-equivalent.

Un sous-ensemble pfaffien d’un ouvert semi-analytique $M\subset {\mathbf{R}}^n$ est une intersection finie d’ensembles semi-analytiques relativement compacts de ${\mathbf{R}}^n$ et de feuilles non spiralantes de certains feuilletages analytiques de codimension 1 de $M.$ Les sous-ensembles semi-pfaffiens de $M$ sont les éléments de la plus petite classe de sous-ensembles de $M$ contenant les sous-ensembles pfaffiens de $M$, stable par intersection finie, réunion finie et différence symétrique. Les ensembles $T$-pfaffiens sont les éléments de la...

Homology with values in a connection with possibly irregular singular points on an algebraic curve is defined, generalizing homology with values in the underlying local system for a connection with regular singular points. Integration defines a perfect pairing between de Rham cohomology with values in the connection and homology with values in the dual connection.

Given an origami (square-tiled surface) $M$ with automorphism group $\Gamma$, we compute the decomposition of the first homology group of $M$ into isotypic $\Gamma$-submodules. Through the action of the affine group of $M$ on the homology group, we deduce some consequences for the multiplicities of the Lyapunov exponents of the Kontsevich-Zorich cocycle. We also construct and study several families of interesting origamis illustrating our results.

Given a Hörmander system $X=\{X_1,\cdots ,X_m\}$ on a domain $\Omega \subseteq {𝐑}^n$ we show that any subelliptic harmonic morphism $\phi$ from $\Omega$ into a $\nu$-dimensional riemannian manifold $N$ is a (smooth) subelliptic harmonic map (in the sense of J. Jost & C-J. Xu, [9]). Also $\phi$ is a submersion provided that $\nu \le m$ and $X$ has rank $m$. If $\Omega ={𝐇}_n$ (the Heisenberg group) and $X=\left\{\frac{1}{2}\left(L_{\alpha }+L_{\overline{\alpha }}\right),\frac{1}{2i}\left(L_{\alpha }-L_{\overline{\alpha }}\right)\right\}$, where $L_{\overline{\alpha }}=\partial /\partial {\overline{z}}^{\alpha }-i{z}^{\alpha }\partial /\partial t$ is the Lewy operator, then a smooth map $\phi :\Omega \to N$ is a subelliptic harmonic morphism if and only if $\phi \circ \pi :(C\left({𝐇}_n\right),F_{{\theta }_0})\to N$ is a harmonic morphism, where $S^1\to C\left({𝐇}_n\right)\stackrel{\pi }{\to }\to {𝐇}_n$ is the canonical circle bundle and $F_{{\theta }_0}$ is the Fefferman...