CTITLEMAINVT -- MATRIX INVERSION                                        00000010
C***********************************************************************
C                 COPYRIGHT ATLANTIC RICHFIELD COMPANY 1991            *
C***********************************************************************
CA    AUTHOR               RALPH MCMILLAN                               00000020
CA    DESIGNER             JIM BROWN                                    00000030
CA    LANGUAGE             S/370 FORTRAN H                              00000040
CA    WRITTEN              01-10-77                                     00000050
C     REVISED                                                           00000060
C                                                                       00000070
CA                                                                      00000080
CA                                                                      00000090
CA    CALL MAINVT (A, B, C, N, E, DEN)                                  00000100
CA         INPUT      A = LOWER DIAGONAL OF TRI-DIAGONAL MATRIX     R4  00000110
CA         INPUT      B = DIAGONAL ELEMENTS OF TRI-DIAGONAL MATRIX  R4  00000120
CA         INPUT      C = UPPER DIAGONAL OF TRI-DIAGONAL MATRIX     R4  00000130
CA         INPUT      N = NUMBER OF ELEMENTS IN A, B, C             I4  00000140
CA         OUTPUT     E = RATIO OF 2 AUX COEFFICIENTS FOR MATRIX    R4  00000150
CA                        SOLUTION                                      00000155
CA         OUTPUT   DEN = AUX ARRAY USED TO SOLVE TRI-DIAGONAL      R4  00000160
CA                        SYSTEM                                        00000165
CA                                                                      00000170
CA                                                                      00000180
CA    THIS ROUTINE PERFORMS A MATRIX INVERSION.                         00000190
CAEND                                                                   00000200
C     EJECT                                                             00000280
C                                                                       00000290
C     LOCAL VARIABLES AND CONSTANTS  (INTERNAL TO SUBROUTINE).          00000300
C                                                                       00000310
C     NM1    = N - 1                                                I4  00000320
C                                                                       00000330
C     EJECT                                                             00000340
      SUBROUTINE MAINVT (A, B, C, N, E, DEN)                            00000350
C                                                                       00000360
      IMPLICIT INTEGER (A-Z)                                            00000370
C                                                                       00000380
C                                                                       00000390
C     REAL ARRAYS IN PARAMETER LIST.                                    00000400
      REAL         A      (1)                                           00000410
      REAL         B      (1)                                           00000420
      REAL         C      (1)                                           00000430
      REAL       DEN      (1)                                           00000440
      REAL         E      (1)                                           00000450
C                                                                       00000460
C                                                                       00000470
      NM1 = N - 1                                                       00000480
C                                                                       00000490
      E(1) = 1.                                                         00000500
C                                                                       00000510
          DO                                                        10  00000520
     *    I = 2, NM1                                                    00000530
          DEN(I) = B(I) + A(I) * E(I-1)                                 00000540
          E(I)   = -C(I) / DEN(I)                                       00000550
C                                                                       00000560
   10     CONTINUE                                                      00000570
C                                                                       00000580
      RETURN                                                            00000590
      END                                                               00000600