Type and cotype with respect to arbitrary orthonormal systems

Abstract

Let $\on_{k \in \nz}$ be an orthonormal system on some $σ$-finite measure space $(\Om,p)$. We study the notion of cotype with respect to $Φ$ for an operator $T$ between two Banach spaces $X$ and $Y$, defined by $\fco T := \inf$ $c$ such that \[ \Tfmm \pl \le \pl c \pll \gmm \hspace{.7cm}\mbox{for all}\hspace{.7cm} (x_k)\subset X \pl,\] where $(g_k)_{k\in \nz}$ is a sequence of independent and normalized gaussian variables. It is shown that this $Φ$-cotype coincides with the usual notion of cotype $2$ iff \linebreak $\fco {I_{\lin}} \sim \sqrt{\frac{n}{\log (n+1)}}$ uniformly in $n$ iff there is a positive $η>0$ such that for all $n \in \nz$ one can find an orthonormal $Ψ= (ψ_l)_1^n \subset {\rm span}\{ ϕ_k \p|\p k \in \nz\}$ and a sequence of disjoint measurable sets $(A_l)_1^n \subset \Om$ with \[ \int\limits_{A_l} \bet ψ_l\rag^2 d p \pl \ge \pl η\quad \mbox{for all}\quad l=1,...,n \pl. \] A similar result holds for the type situation. The study of type and cotype with respect to orthonormal systems of a given length provides the appropriate approach to this result. We intend to give a quite complete picture for orthonormal systems in measure space with few atoms.