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81 lines
2.4 KiB
Fortran
81 lines
2.4 KiB
Fortran
*DECK SPPSL
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SUBROUTINE SPPSL (AP, N, B)
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C***BEGIN PROLOGUE SPPSL
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C***PURPOSE Solve the real symmetric positive definite system using
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C the factors computed by SPPCO or SPPFA.
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C***LIBRARY SLATEC (LINPACK)
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C***CATEGORY D2B1B
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C***TYPE SINGLE PRECISION (SPPSL-S, DPPSL-D, CPPSL-C)
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C***KEYWORDS LINEAR ALGEBRA, LINPACK, MATRIX, PACKED,
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C POSITIVE DEFINITE, SOLVE
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C***AUTHOR Moler, C. B., (U. of New Mexico)
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C***DESCRIPTION
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C
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C SPPSL solves the real symmetric positive definite system
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C A * X = B
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C using the factors computed by SPPCO or SPPFA.
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C
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C On Entry
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C
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C AP REAL (N*(N+1)/2)
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C the output from SPPCO or SPPFA.
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C
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C N INTEGER
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C the order of the matrix A .
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C
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C B REAL(N)
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C the right hand side vector.
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C
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C On Return
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C
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C B the solution vector X .
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C
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C Error Condition
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C
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C A division by zero will occur if the input factor contains
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C a zero on the diagonal. Technically, this indicates
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C singularity, but it is usually caused by improper subroutine
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C arguments. It will not occur if the subroutines are called
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C correctly and INFO .EQ. 0 .
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C
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C To compute INVERSE(A) * C where C is a matrix
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C with P columns
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C CALL SPPCO(AP,N,RCOND,Z,INFO)
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C IF (RCOND is too small .OR. INFO .NE. 0) GO TO ...
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C DO 10 J = 1, P
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C CALL SPPSL(AP,N,C(1,J))
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C 10 CONTINUE
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C
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C***REFERENCES J. J. Dongarra, J. R. Bunch, C. B. Moler, and G. W.
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C Stewart, LINPACK Users' Guide, SIAM, 1979.
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C***ROUTINES CALLED SAXPY, SDOT
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C***REVISION HISTORY (YYMMDD)
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C 780814 DATE WRITTEN
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C 890831 Modified array declarations. (WRB)
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C 890831 REVISION DATE from Version 3.2
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C 891214 Prologue converted to Version 4.0 format. (BAB)
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C 900326 Removed duplicate information from DESCRIPTION section.
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C (WRB)
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C 920501 Reformatted the REFERENCES section. (WRB)
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C***END PROLOGUE SPPSL
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INTEGER N
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REAL AP(*),B(*)
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C
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REAL SDOT,T
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INTEGER K,KB,KK
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C***FIRST EXECUTABLE STATEMENT SPPSL
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KK = 0
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DO 10 K = 1, N
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T = SDOT(K-1,AP(KK+1),1,B(1),1)
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KK = KK + K
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B(K) = (B(K) - T)/AP(KK)
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10 CONTINUE
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DO 20 KB = 1, N
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K = N + 1 - KB
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B(K) = B(K)/AP(KK)
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KK = KK - K
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T = -B(K)
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CALL SAXPY(K-1,T,AP(KK+1),1,B(1),1)
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20 CONTINUE
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RETURN
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END
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