= Coherence of a normalized matrix
{title2=$\mu(A)=\max_{i\ne j}|\langle a_i,a_j\rangle|$}
= Mutual coherence
{synonym}
For a <matrix> with at least two columns of unit <Euclidean norm>, its mutual coherence is the largest magnitude of an <inner product> between distinct columns. It measures the strongest pairwise ambiguity of linear measurements. The <Cauchy-Schwarz inequality> gives $0\le\mu\le1$. The order-two <restricted isometry constant> equals the <mutual coherence>, because every two-column <Gram matrix> has eigenvalues $1\pm|\langle a_i,a_j\rangle|$.
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