= Discrete isotropic total variation
{title2=$\|Du\|_{2,1}$}
With zero forward differences at the grid boundary, discrete isotropic total variation is the sum of the Euclidean lengths of the two-component forward gradients. Its dual constraint is the product of unit Euclidean balls, and minimizing $\tfrac12\|g-D^*p\|_2^2$ over that product yields $u_*=g-D^*p_*$. The exact difference <adjoint operator> is essential. A dual <proximal gradient method> projects $p+\tau D(g-D^*p)$ pointwise onto unit balls; $0<\tau\le1/8$ is safe on an unscaled square grid.
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