[Octave-bug-tracker] [bug #47415] out of memory with factorized sparse matrix using mldivide

[John W. Eaton]

Follow-up Comment #5, bug #47415 (project octave):

The problem isn't the multiplication or division steps, it is the use of the
unary minus operator on jacQ.  Your single line expression isn't exactly the
same as the series of operations.  To be equivalent, you need to write


delta = - (jacQ * (jacU \ (jacL \ (jacP * (jacR \ res)))));


You will also see the same error if you write


-jacQ


The reason is that jacQ is a permutation matrix, not a sparse matrix, and for
permutation matrices, negation returns -0 for all the 0 elements of the
original permutation matrix and that forces it to be converted to a full
matrix.  For a sparse matrix, the unary minus operator ignores the 0
elements.

I'm not sure what the best solution is.  I suppose the real question is
whether the following should produce the same result for a permutation matrix
P:


-full (P)
full (-P)


Currently this is true for permutation matrices, but not for sparse matrices.

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