Block coordinate gradient
= Block coordinate gradient
{title2=$\nabla_{\mathcal B}f(x)=\sum_{i\in\mathcal B}\partial_i f(x)e_i$}
The <orthogonal projection> of the <gradient> onto the <coordinate subspace> indexed by $\mathcal B$. It is kept as a <vector> in the original <Euclidean space>, with zero coordinates outside the block. For a singleton block it is $\partial_i f(x)e_i$; for the full block it is $\nabla f(x)$.
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Target: gradient-descent.bigb, under Lipschitz gradient.
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