146 lines
4.1 KiB
Fortran
146 lines
4.1 KiB
Fortran
SUBROUTINE DTRTI2F( UPLO, DIAG, N, A, LDA, INFO )
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*
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* -- LAPACK routine (version 3.1) --
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* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
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* November 2006
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*
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* .. Scalar Arguments ..
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CHARACTER DIAG, UPLO
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INTEGER INFO, LDA, N
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* ..
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* .. Array Arguments ..
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DOUBLE PRECISION A( LDA, * )
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* ..
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*
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* Purpose
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* =======
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*
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* DTRTI2 computes the inverse of a real upper or lower triangular
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* matrix.
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*
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* This is the Level 2 BLAS version of the algorithm.
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*
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* Arguments
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* =========
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*
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* UPLO (input) CHARACTER*1
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* Specifies whether the matrix A is upper or lower triangular.
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* = 'U': Upper triangular
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* = 'L': Lower triangular
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*
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* DIAG (input) CHARACTER*1
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* Specifies whether or not the matrix A is unit triangular.
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* = 'N': Non-unit triangular
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* = 'U': Unit triangular
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*
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* N (input) INTEGER
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* The order of the matrix A. N >= 0.
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*
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* A (input/output) DOUBLE PRECISION array, dimension (LDA,N)
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* On entry, the triangular matrix A. If UPLO = 'U', the
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* leading n by n upper triangular part of the array A contains
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* the upper triangular matrix, and the strictly lower
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* triangular part of A is not referenced. If UPLO = 'L', the
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* leading n by n lower triangular part of the array A contains
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* the lower triangular matrix, and the strictly upper
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* triangular part of A is not referenced. If DIAG = 'U', the
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* diagonal elements of A are also not referenced and are
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* assumed to be 1.
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*
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* On exit, the (triangular) inverse of the original matrix, in
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* the same storage format.
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*
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* LDA (input) INTEGER
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* The leading dimension of the array A. LDA >= max(1,N).
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*
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* INFO (output) INTEGER
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* = 0: successful exit
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* < 0: if INFO = -k, the k-th argument had an illegal value
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*
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* =====================================================================
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*
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* .. Parameters ..
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DOUBLE PRECISION ONE
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PARAMETER ( ONE = 1.0D+0 )
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* ..
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* .. Local Scalars ..
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LOGICAL NOUNIT, UPPER
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INTEGER J
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DOUBLE PRECISION AJJ
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* ..
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* .. External Functions ..
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LOGICAL LSAME
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EXTERNAL LSAME
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* ..
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* .. External Subroutines ..
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EXTERNAL DSCAL, DTRMV, XERBLA
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* ..
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* .. Intrinsic Functions ..
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INTRINSIC MAX
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* ..
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* .. Executable Statements ..
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*
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* Test the input parameters.
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*
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INFO = 0
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UPPER = LSAME( UPLO, 'U' )
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NOUNIT = LSAME( DIAG, 'N' )
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IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
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INFO = -1
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ELSE IF( .NOT.NOUNIT .AND. .NOT.LSAME( DIAG, 'U' ) ) THEN
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INFO = -2
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ELSE IF( N.LT.0 ) THEN
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INFO = -3
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ELSE IF( LDA.LT.MAX( 1, N ) ) THEN
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INFO = -5
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END IF
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IF( INFO.NE.0 ) THEN
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CALL XERBLA( 'DTRTI2', -INFO )
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RETURN
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END IF
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*
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IF( UPPER ) THEN
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*
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* Compute inverse of upper triangular matrix.
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*
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DO 10 J = 1, N
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IF( NOUNIT ) THEN
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A( J, J ) = ONE / A( J, J )
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AJJ = -A( J, J )
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ELSE
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AJJ = -ONE
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END IF
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*
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* Compute elements 1:j-1 of j-th column.
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*
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CALL DTRMV( 'Upper', 'No transpose', DIAG, J-1, A, LDA,
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$ A( 1, J ), 1 )
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CALL DSCAL( J-1, AJJ, A( 1, J ), 1 )
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10 CONTINUE
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ELSE
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*
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* Compute inverse of lower triangular matrix.
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*
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DO 20 J = N, 1, -1
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IF( NOUNIT ) THEN
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A( J, J ) = ONE / A( J, J )
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AJJ = -A( J, J )
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ELSE
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AJJ = -ONE
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END IF
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IF( J.LT.N ) THEN
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*
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* Compute elements j+1:n of j-th column.
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*
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CALL DTRMV( 'Lower', 'No transpose', DIAG, N-J,
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$ A( J+1, J+1 ), LDA, A( J+1, J ), 1 )
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CALL DSCAL( N-J, AJJ, A( J+1, J ), 1 )
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END IF
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20 CONTINUE
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END IF
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*
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RETURN
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*
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* End of DTRTI2
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*
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END
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