Metric properties in the mean of polynomials on compact isotropy irreducible homogeneous spaces

Abstract

Let $M=G/H$ be a compact connected isotropy irreducible Riemannian homogeneous manifold, where $G$ is a compact Lie group (may be, disconnected) acting on $M$ by isometries. This class includes all compact irreducible Riemannian symmetric spaces and, for example, the tori $\bbR^n/\bbZ^n$ with the natural action on itself extended by the finite group generated by all transpositions of coordinates and inversions in circle factors. We say that $u$ is a polynomial on $M$ if it belongs to some $G$-invariant finite dimensional subspace $\cE$ of $L^2(M)$. We compute or estimate from above the averages over the unit sphere $\cS$ in $\cE$ for some metric quantities such as Hausdorff measures of level set and norms in $L^p(M)$, $1\leq p\leq\infty$, where $M$ is equipped with the invariant probability measure. For example, the averages over $\cS$ of $\|u\|_{L^p(M)}$, $p\geq2$, are less than $\sqrt{\frac{p+1}{e}}$ independently of $M$ and $\cE$.