[petsc-users] Matrix is missing diagonal entry

[Jed Brown]

"Kong, Fande" <fande.kong at inl.gov> writes:

> Hi Developers,
>
> Any reason to force users provide a matrix which does not miss any diagonal
> entries when using a LU-type solver?

Automatically adding the entries for incomplete factorization is a
slight increase in complexity and means that "ILU(0)" actually has some
fill.  Note that a literal ILU(0) cannot work if there are missing
diagonal entries.

It's probably not a significant code complexity difference for LU.

> Sometime, it is impossible to have all diagonal entries in a matrix, that
> is, the matrix has to miss some diagonal entries. For example, there is a
> saddle-point matrix from the discretization of incomprehensible equations,
> and the lower part of the matrix is a zero block. The matrix usually looks
> like:
>
> | A   B^T  |
> | B    0     |

Just insert explicit zeros on the diagonal.
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