Restricted isometry constant
= Restricted isometry constant
{title2=$\delta_s(A)$}
The order-s restricted isometry constant is the smallest nonnegative $\delta$ satisfying $(1-\delta)\|x\|_2^2\le\|Ax\|_2^2\le(1+\delta)\|x\|_2^2$ for every vector with at most $s$ nonzero coordinates. Equivalently it is $\max_{|S|\le s}\|A_S^*A_S-I\|_{2\to2}$. The sparse-vector quantifier is essential, and the constant can equal zero.