Diagonal Elements of an Inverse Matrix

[Zhifeng Sheng]

Tim Stitt wrote:

> Hi PETSc Users/Developers,
>
> I was just wondering if anyone knew of any O(N) methods for obtaining 
> the diagonal elements of the inverse of a block tridiagonal 
> matrix,without computing all the off-diagonal values at the same time?
>
> Actually, the general case would be most useful were selected elements 
> in the inverse could be obtained in O(N) time.
>
> I would be grateful if anyone could shed any light on this...
>
> Thanks,
>
> Tim.
>
There is one method call approximate schur inverse . It computes the 
exact number of inverse matrix on diagonals without knowing the number on

off-diagonal blocks.

best regards
Zhifeng