[petsc-users] forming a matrix from a set of vectors

[Jed Brown]

Nicolas Pozin <nicolas.pozin at inria.fr> writes:

> Actually I want to get the diagonal of the matrix : transpose(d)*A*d where
> -d is a sparse matrix of size (n1,m1)
> -A is a dense symetric matrix of size size (n1,n1)
> with m1 very big compared to n1 (1 million against a few dozens).

The result will be m1 × m1, but at most rank n1.  Why would you want
that monster as a matrix?
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