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86 lines
2.7 KiB
Fortran
86 lines
2.7 KiB
Fortran
*DECK DGBDI
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SUBROUTINE DGBDI (ABD, LDA, N, ML, MU, IPVT, DET)
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C***BEGIN PROLOGUE DGBDI
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C***PURPOSE Compute the determinant of a band matrix using the factors
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C computed by DGBCO or DGBFA.
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C***LIBRARY SLATEC (LINPACK)
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C***CATEGORY D3A2
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C***TYPE DOUBLE PRECISION (SGBDI-S, DGBDI-D, CGBDI-C)
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C***KEYWORDS BANDED, DETERMINANT, INVERSE, LINEAR ALGEBRA, LINPACK,
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C MATRIX
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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 DGBDI computes the determinant of a band matrix
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C using the factors computed by DGBCO or DGBFA.
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C If the inverse is needed, use DGBSL N times.
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C
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C On Entry
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C
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C ABD DOUBLE PRECISION(LDA, N)
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C the output from DGBCO or DGBFA.
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C
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C LDA INTEGER
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C the leading dimension of the array ABD .
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C
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C N INTEGER
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C the order of the original matrix.
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C
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C ML INTEGER
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C number of diagonals below the main diagonal.
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C
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C MU INTEGER
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C number of diagonals above the main diagonal.
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C
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C IPVT INTEGER(N)
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C the pivot vector from DGBCO or DGBFA.
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C
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C On Return
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C
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C DET DOUBLE PRECISION(2)
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C determinant of original matrix.
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C Determinant = DET(1) * 10.0**DET(2)
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C with 1.0 .LE. ABS(DET(1)) .LT. 10.0
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C or DET(1) = 0.0 .
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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 (NONE)
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C***REVISION HISTORY (YYMMDD)
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C 780814 DATE WRITTEN
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C 890531 Changed all specific intrinsics to generic. (WRB)
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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 DGBDI
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INTEGER LDA,N,ML,MU,IPVT(*)
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DOUBLE PRECISION ABD(LDA,*),DET(2)
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C
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DOUBLE PRECISION TEN
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INTEGER I,M
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C***FIRST EXECUTABLE STATEMENT DGBDI
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M = ML + MU + 1
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DET(1) = 1.0D0
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DET(2) = 0.0D0
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TEN = 10.0D0
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DO 50 I = 1, N
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IF (IPVT(I) .NE. I) DET(1) = -DET(1)
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DET(1) = ABD(M,I)*DET(1)
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IF (DET(1) .EQ. 0.0D0) GO TO 60
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10 IF (ABS(DET(1)) .GE. 1.0D0) GO TO 20
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DET(1) = TEN*DET(1)
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DET(2) = DET(2) - 1.0D0
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GO TO 10
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20 CONTINUE
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30 IF (ABS(DET(1)) .LT. TEN) GO TO 40
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DET(1) = DET(1)/TEN
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DET(2) = DET(2) + 1.0D0
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GO TO 30
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40 CONTINUE
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50 CONTINUE
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60 CONTINUE
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RETURN
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END
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