= L0 sparsity count
{c}
{title2=$\|x\|_0=|\operatorname{supp}x|$}
The number of nonzero coordinates of a finite <vector> is its L0 sparsity count. It vanishes only at zero, but is unchanged under multiplication by a nonzero scalar, so it is not a <norm>. Minimizing it under linear measurement constraints finds a <sparse vector> with the smallest possible <support of a vector>. <Sparse injectivity> of order $s$ guarantees unique recovery of every <sparse vector> with at most $s$ nonzero coordinates by this objective.
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