VHS3

C
C                      * * * * * * * * * * * * *
C                      *                       *
C                      *          VHS3         *
C                      *                       *
C                      * * * * * * * * * * * * *
C
C
C                       (VERSION 1, AUGUST 1985)
C
C     A VECTORIZED PACKAGE OF FORTRAN SUBPROGRAMS FOR THE SOLUTION
C     OF A THREE-DIMENSIONAL HELMHOLTZ EQUATION ON A STAGGERED GRID
C
C                                  BY
C
C                           ROLAND A. SWEET
C                    SCIENTIFIC COMPUTING DIVISION
C                     NATIONAL BUREAU OF STANDARDS
C                       BOULDER, COLORADO 80303
C
C
C                       * * * * * * * * * * * * *
C                       *                       *
C                       *       PURPOSE         *
C                       *                       *
C                       * * * * * * * * * * * * *
C
C
C     THIS PACKAGE SOLVES THE STANDARD SEVEN-POINT FINITE DIFFERENCE
C  APPROXIMATION ON A STAGGERED GRID TO THE HELMHOLTZ EQUATION IN
C  CARTESIAN COORDINATES
C
C     (D/DX)(DU/DX)+(D/DY)(DU/DY)+(D/DZ)(DU/DZ) + LAMBDA*U = F(X,Y,Z)
C
C  WITH A VARIETY OF POSSIBLE BOUNDARY CONDITIONS.
C
C
C                * * * * * * * * * * * * * * * * * * * *
C                *                                     *
C                *       DESCRIPTION OF PACKAGE        *
C                *                                     *
C                * * * * * * * * * * * * * * * * * * * *
C
C
C     THIS PACKAGE CONSISTS OF FOUR USER-CALLABLE FORTRAN SUBROUTINES
C  FOR THE SOLUTION OF FINITE DIFFERENCE APPROXIMATIONS ON A STAGGERED
C  (CELL-CENTERED) GRID TO THREE-DIMENSIONAL HELMHOLTZ EQUATIONS.
C
C     THE USER-CALLABLE SUBROUTINES ARE:
C
C     1.  HS3CRI     INITIALIZATION ROUTINE FOR HS3CRT
C     2.  HS3CRT     GENERATES A FINITE DIFFERENCE APPROXIMATION ON
C                    A STAGGERED GRID FOR THE HELMHOLTZ EQUATION IN
C                    THREE DIMENSIONS AND SOLVES IT BY CALLING PSTG3D
C     3.  PSTG3I     INITIALIZATION ROUTINE FOR PSTG3D
C     4.  PSTG3D     SOLVES CERTAIN BLOCK TRIDIAGONAL SYSTEMS OF LINEAR
C                    ALGEBRAIC EQUATIONS ARISING FROM THE DISCRETIZATION
C                    OF THE THREE-DIMENSIONAL HELMHOLTZ EQUATION ON A
C                    STAGGERED GRID
C