Algorithm 603: COLROW and ARCECO: FORTRAN Packages for Solving Certain Almost Block Diagonal Linear Systems by Modified Alternate Row and Column Elimination

In the numerical solution of boundary-value problems for ordinary differential equations, hnear systems with a partmular block structure are encountered. In this paper two packages of FORTRAN subroutines, COLROW and ARCECO, which use alternate row and column elimination for solving such systems, as discussed by Varah, are described Varah's procedure is stable, and introduces no fillin, tha t is, reqmres no additional storage To improve its efficiency, use is made of the fact tha t after each sequence of eliminations a reducible matrix is obtained. In addition, the packages presented treat systems with a more general structure than those considered by Varah. The results of numerical experiments, which demonstrate the effectiveness of the new packages and their superiority over a prevmusly pubhshed code, on a suitably restrmted class of problems, are presented