SUBROUTINE DSYTRS( UPLO, N, NRHS, A, LDA, IPIV, B, LDB, INFO ) * * -- LAPACK routine (version 3.3.1) -- * -- LAPACK is a software package provided by Univ. of Tennessee, -- * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..-- * -- April 2011 -- * * .. Scalar Arguments .. CHARACTER UPLO INTEGER INFO, LDA, LDB, N, NRHS * .. * .. Array Arguments .. INTEGER IPIV( * ) DOUBLE PRECISION A( LDA, * ), B( LDB, * ) * .. * * Purpose * ======= * * DSYTRS solves a system of linear equations A*X = B with a real * symmetric matrix A using the factorization A = U*D*U**T or * A = L*D*L**T computed by DSYTRF. * * Arguments * ========= * * UPLO (input) CHARACTER*1 * Specifies whether the details of the factorization are stored * as an upper or lower triangular matrix. * = 'U': Upper triangular, form is A = U*D*U**T; * = 'L': Lower triangular, form is A = L*D*L**T. * * N (input) INTEGER * The order of the matrix A. N >= 0. * * NRHS (input) INTEGER * The number of right hand sides, i.e., the number of columns * of the matrix B. NRHS >= 0. * * A (input) DOUBLE PRECISION array, dimension (LDA,N) * The block diagonal matrix D and the multipliers used to * obtain the factor U or L as computed by DSYTRF. * * LDA (input) INTEGER * The leading dimension of the array A. LDA >= max(1,N). * * IPIV (input) INTEGER array, dimension (N) * Details of the interchanges and the block structure of D * as determined by DSYTRF. * * B (input/output) DOUBLE PRECISION array, dimension (LDB,NRHS) * On entry, the right hand side matrix B. * On exit, the solution matrix X. * * LDB (input) INTEGER * The leading dimension of the array B. LDB >= max(1,N). * * INFO (output) INTEGER * = 0: successful exit * < 0: if INFO = -i, the i-th argument had an illegal value * * ===================================================================== * * .. Parameters .. DOUBLE PRECISION ONE PARAMETER ( ONE = 1.0D+0 ) * .. * .. Local Scalars .. LOGICAL UPPER INTEGER J, K, KP DOUBLE PRECISION AK, AKM1, AKM1K, BK, BKM1, DENOM * .. * .. External Functions .. LOGICAL LSAME EXTERNAL LSAME * .. * .. External Subroutines .. EXTERNAL DGEMV, DGER, DSCAL, DSWAP, XERBLA * .. * .. Intrinsic Functions .. INTRINSIC MAX * .. * .. Executable Statements .. * INFO = 0 UPPER = LSAME( UPLO, 'U' ) IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN INFO = -1 ELSE IF( N.LT.0 ) THEN INFO = -2 ELSE IF( NRHS.LT.0 ) THEN INFO = -3 ELSE IF( LDA.LT.MAX( 1, N ) ) THEN INFO = -5 ELSE IF( LDB.LT.MAX( 1, N ) ) THEN INFO = -8 END IF IF( INFO.NE.0 ) THEN CALL XERBLA( 'DSYTRS', -INFO ) RETURN END IF * * Quick return if possible * IF( N.EQ.0 .OR. NRHS.EQ.0 ) \$ RETURN * IF( UPPER ) THEN * * Solve A*X = B, where A = U*D*U**T. * * First solve U*D*X = B, overwriting B with X. * * K is the main loop index, decreasing from N to 1 in steps of * 1 or 2, depending on the size of the diagonal blocks. * K = N 10 CONTINUE * * If K < 1, exit from loop. * IF( K.LT.1 ) \$ GO TO 30 * IF( IPIV( K ).GT.0 ) THEN * * 1 x 1 diagonal block * * Interchange rows K and IPIV(K). * KP = IPIV( K ) IF( KP.NE.K ) \$ CALL DSWAP( NRHS, B( K, 1 ), LDB, B( KP, 1 ), LDB ) * * Multiply by inv(U(K)), where U(K) is the transformation * stored in column K of A. * CALL DGER( K-1, NRHS, -ONE, A( 1, K ), 1, B( K, 1 ), LDB, \$ B( 1, 1 ), LDB ) * * Multiply by the inverse of the diagonal block. * CALL DSCAL( NRHS, ONE / A( K, K ), B( K, 1 ), LDB ) K = K - 1 ELSE * * 2 x 2 diagonal block * * Interchange rows K-1 and -IPIV(K). * KP = -IPIV( K ) IF( KP.NE.K-1 ) \$ CALL DSWAP( NRHS, B( K-1, 1 ), LDB, B( KP, 1 ), LDB ) * * Multiply by inv(U(K)), where U(K) is the transformation * stored in columns K-1 and K of A. * CALL DGER( K-2, NRHS, -ONE, A( 1, K ), 1, B( K, 1 ), LDB, \$ B( 1, 1 ), LDB ) CALL DGER( K-2, NRHS, -ONE, A( 1, K-1 ), 1, B( K-1, 1 ), \$ LDB, B( 1, 1 ), LDB ) * * Multiply by the inverse of the diagonal block. * AKM1K = A( K-1, K ) AKM1 = A( K-1, K-1 ) / AKM1K AK = A( K, K ) / AKM1K DENOM = AKM1*AK - ONE DO 20 J = 1, NRHS BKM1 = B( K-1, J ) / AKM1K BK = B( K, J ) / AKM1K B( K-1, J ) = ( AK*BKM1-BK ) / DENOM B( K, J ) = ( AKM1*BK-BKM1 ) / DENOM 20 CONTINUE K = K - 2 END IF * GO TO 10 30 CONTINUE * * Next solve U**T *X = B, overwriting B with X. * * K is the main loop index, increasing from 1 to N in steps of * 1 or 2, depending on the size of the diagonal blocks. * K = 1 40 CONTINUE * * If K > N, exit from loop. * IF( K.GT.N ) \$ GO TO 50 * IF( IPIV( K ).GT.0 ) THEN * * 1 x 1 diagonal block * * Multiply by inv(U**T(K)), where U(K) is the transformation * stored in column K of A. * CALL DGEMV( 'Transpose', K-1, NRHS, -ONE, B, LDB, A( 1, K ), \$ 1, ONE, B( K, 1 ), LDB ) * * Interchange rows K and IPIV(K). * KP = IPIV( K ) IF( KP.NE.K ) \$ CALL DSWAP( NRHS, B( K, 1 ), LDB, B( KP, 1 ), LDB ) K = K + 1 ELSE * * 2 x 2 diagonal block * * Multiply by inv(U**T(K+1)), where U(K+1) is the transformation * stored in columns K and K+1 of A. * CALL DGEMV( 'Transpose', K-1, NRHS, -ONE, B, LDB, A( 1, K ), \$ 1, ONE, B( K, 1 ), LDB ) CALL DGEMV( 'Transpose', K-1, NRHS, -ONE, B, LDB, \$ A( 1, K+1 ), 1, ONE, B( K+1, 1 ), LDB ) * * Interchange rows K and -IPIV(K). * KP = -IPIV( K ) IF( KP.NE.K ) \$ CALL DSWAP( NRHS, B( K, 1 ), LDB, B( KP, 1 ), LDB ) K = K + 2 END IF * GO TO 40 50 CONTINUE * ELSE * * Solve A*X = B, where A = L*D*L**T. * * First solve L*D*X = B, overwriting B with X. * * K is the main loop index, increasing from 1 to N in steps of * 1 or 2, depending on the size of the diagonal blocks. * K = 1 60 CONTINUE * * If K > N, exit from loop. * IF( K.GT.N ) \$ GO TO 80 * IF( IPIV( K ).GT.0 ) THEN * * 1 x 1 diagonal block * * Interchange rows K and IPIV(K). * KP = IPIV( K ) IF( KP.NE.K ) \$ CALL DSWAP( NRHS, B( K, 1 ), LDB, B( KP, 1 ), LDB ) * * Multiply by inv(L(K)), where L(K) is the transformation * stored in column K of A. * IF( K.LT.N ) \$ CALL DGER( N-K, NRHS, -ONE, A( K+1, K ), 1, B( K, 1 ), \$ LDB, B( K+1, 1 ), LDB ) * * Multiply by the inverse of the diagonal block. * CALL DSCAL( NRHS, ONE / A( K, K ), B( K, 1 ), LDB ) K = K + 1 ELSE * * 2 x 2 diagonal block * * Interchange rows K+1 and -IPIV(K). * KP = -IPIV( K ) IF( KP.NE.K+1 ) \$ CALL DSWAP( NRHS, B( K+1, 1 ), LDB, B( KP, 1 ), LDB ) * * Multiply by inv(L(K)), where L(K) is the transformation * stored in columns K and K+1 of A. * IF( K.LT.N-1 ) THEN CALL DGER( N-K-1, NRHS, -ONE, A( K+2, K ), 1, B( K, 1 ), \$ LDB, B( K+2, 1 ), LDB ) CALL DGER( N-K-1, NRHS, -ONE, A( K+2, K+1 ), 1, \$ B( K+1, 1 ), LDB, B( K+2, 1 ), LDB ) END IF * * Multiply by the inverse of the diagonal block. * AKM1K = A( K+1, K ) AKM1 = A( K, K ) / AKM1K AK = A( K+1, K+1 ) / AKM1K DENOM = AKM1*AK - ONE DO 70 J = 1, NRHS BKM1 = B( K, J ) / AKM1K BK = B( K+1, J ) / AKM1K B( K, J ) = ( AK*BKM1-BK ) / DENOM B( K+1, J ) = ( AKM1*BK-BKM1 ) / DENOM 70 CONTINUE K = K + 2 END IF * GO TO 60 80 CONTINUE * * Next solve L**T *X = B, overwriting B with X. * * K is the main loop index, decreasing from N to 1 in steps of * 1 or 2, depending on the size of the diagonal blocks. * K = N 90 CONTINUE * * If K < 1, exit from loop. * IF( K.LT.1 ) \$ GO TO 100 * IF( IPIV( K ).GT.0 ) THEN * * 1 x 1 diagonal block * * Multiply by inv(L**T(K)), where L(K) is the transformation * stored in column K of A. * IF( K.LT.N ) \$ CALL DGEMV( 'Transpose', N-K, NRHS, -ONE, B( K+1, 1 ), \$ LDB, A( K+1, K ), 1, ONE, B( K, 1 ), LDB ) * * Interchange rows K and IPIV(K). * KP = IPIV( K ) IF( KP.NE.K ) \$ CALL DSWAP( NRHS, B( K, 1 ), LDB, B( KP, 1 ), LDB ) K = K - 1 ELSE * * 2 x 2 diagonal block * * Multiply by inv(L**T(K-1)), where L(K-1) is the transformation * stored in columns K-1 and K of A. * IF( K.LT.N ) THEN CALL DGEMV( 'Transpose', N-K, NRHS, -ONE, B( K+1, 1 ), \$ LDB, A( K+1, K ), 1, ONE, B( K, 1 ), LDB ) CALL DGEMV( 'Transpose', N-K, NRHS, -ONE, B( K+1, 1 ), \$ LDB, A( K+1, K-1 ), 1, ONE, B( K-1, 1 ), \$ LDB ) END IF * * Interchange rows K and -IPIV(K). * KP = -IPIV( K ) IF( KP.NE.K ) \$ CALL DSWAP( NRHS, B( K, 1 ), LDB, B( KP, 1 ), LDB ) K = K - 2 END IF * GO TO 90 100 CONTINUE END IF * RETURN * * End of DSYTRS * END