In block factorizations a pivot block is generally forced to be sparse, typically of banded form, and that we need an approximation to its inverse that has a similar structure. Furthermore, this approximation should be easily computable, so we rule out the option of calculating the full inverse and taking a banded part of it.

The simplest approximation to is the diagonal matrix of the reciprocals of the diagonal of : .

**Figure:** Algorithm for approximating the inverse of a banded matrix

Other possibilities were considered by Axelsson and Eijkhout [15], Axelsson and Polman [21], Concus, Golub and Meurant [57], Eijkhout and Vassilevski [90], Kolotilina and Yeremin [141], and Meurant [155]. One particular example is given in figure . It has the attractive theoretical property that, if the original matrix is symmetric positive definite and a factorization with positive diagonal can be made, the approximation to the inverse is again symmetric positive definite.

Banded approximations to the inverse of banded matrices have a theoretical justification. In the context of partial differential equations the diagonal blocks of the coefficient matrix are usually strongly diagonally dominant. For such matrices, the elements of the inverse have a size that is exponentially decreasing in their distance from the main diagonal. See Demko, Moss and Smith [65] for a general proof, and Eijkhout and Polman [89] for a more detailed analysis in the -matrix case.

Mon Nov 20 08:52:54 EST 1995