Ramsey-like properties for bi-Lipschitz mappings of finite metric spaces

summary:Let $(X,\rho)$, $(Y,\sigma)$ be metric spaces and $f:X\to Y$ an injective mapping. We put $\|f\|_{Lip} = \sup \{\sigma (f(x),f(y))/\rho(x,y)$; $x,y\in X$, $x\neq y\}$, and $\operatorname{dist}(f)= \|f\|_{Lip}.\|f^{-1}\|_{Lip}$ (the {\sl distortion\/} of the mapping $f$). Some Ramsey-type questions for mappings of finite metric spaces with bounded distortion are studied; e.g., the following theorem is proved: Let $X$ be a finite metric space, and let $\varepsilon>0$, $K$ be given numbers. Then there exists a finite metric space $Y$, such that for every mapping $f:Y\to Z$ ($Z$ arbitrary metric space) with $\operatorname{dist}(f)<K$ one can find a mapping $g:X\to Y$, such that both the mappings $g$ and $f|_{g(X)}$ have distortion at most $(1+\varepsilon)$. If $X$ is isometrically embeddable into a $\ell_p$ space (for some $p\in [1,\infty]$), then also $Y$ can be chosen with this property

Some remarks on harmonic projection operators on spheres

We give a survey of recent works concerning the mapping properties of joint harmonic projection operators, mapping the space of square integrable functions on complex and quaternionic spheres onto the eigenspaces of the Laplace-Beltrami operator and of a suitably defined subLaplacian. In particular, we discuss similarities and differences between the real, the complex and the quaternionic framework