Cotype of a Banach space
= Cotype of a Banach space
{title2=$C_q(X)$}
A <Banach space> $X$ has cotype $q$, $2\le q<\infty$, if a constant $C$ independent of the number of vectors satisfies $(\sum_j\|x_j\|^q)^{1/q}\le C(\mathbb E\|\sum_j\varepsilon_jx_j\|^2)^{1/2}$ for independent symmetric <Rademacher random variables>. It prevents many large vectors from having a small random sum.