SUBROUTINE DPTTRSV( TRANS, N, NRHS, D, E, B, LDB, $ INFO ) * * Written by Andrew J. Cleary, University of Tennessee. * November, 1996. * Modified from DPTTRS: * -- LAPACK routine (preliminary version) -- * Univ. of Tennessee, Univ. of California Berkeley, NAG Ltd., * Courant Institute, Argonne National Lab, and Rice University * * .. Scalar Arguments .. CHARACTER TRANS INTEGER INFO, LDB, N, NRHS * .. * .. Array Arguments .. DOUBLE PRECISION D( * ) DOUBLE PRECISION B( LDB, * ), E( * ) * .. * * Purpose * ======= * * DPTTRSV solves one of the triangular systems * L**T* X = B, or L * X = B, * where L is the Cholesky factor of a Hermitian positive * definite tridiagonal matrix A such that * A = L*D*L**H (computed by DPTTRF). * * Arguments * ========= * * TRANS (input) CHARACTER * Specifies the form of the system of equations: * = 'N': L * X = B (No transpose) * = 'T': L**T * X = B (Transpose) * * N (input) INTEGER * The order of the tridiagonal 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. * * D (input) REAL array, dimension (N) * The n diagonal elements of the diagonal matrix D from the * factorization computed by DPTTRF. * * E (input) COMPLEX array, dimension (N-1) * The (n-1) off-diagonal elements of the unit bidiagonal * factor U or L from the factorization computed by DPTTRF * (see UPLO). * * B (input/output) COMPLEX 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 * * ===================================================================== * * .. Local Scalars .. LOGICAL NOTRAN INTEGER I, J * .. * .. External Functions .. LOGICAL LSAME EXTERNAL LSAME * .. * .. External Subroutines .. EXTERNAL XERBLA * .. * .. Intrinsic Functions .. INTRINSIC MAX * .. * .. Executable Statements .. * * Test the input arguments. * INFO = 0 NOTRAN = LSAME( TRANS, 'N' ) IF( .NOT.NOTRAN .AND. .NOT.LSAME( TRANS, 'T' ) ) THEN INFO = -1 ELSE IF( N.LT.0 ) THEN INFO = -2 ELSE IF( NRHS.LT.0 ) THEN INFO = -3 ELSE IF( LDB.LT.MAX( 1, N ) ) THEN INFO = -7 END IF IF( INFO.NE.0 ) THEN CALL XERBLA( 'DPTTRS', -INFO ) RETURN END IF * * Quick return if possible * IF( N.EQ.0 ) $ RETURN IF( NOTRAN ) THEN * DO 60 J = 1, NRHS * * Solve L * x = b. * DO 40 I = 2, N B( I, J ) = B( I, J ) - B( I-1, J )*E( I-1 ) 40 CONTINUE 60 CONTINUE * ELSE * DO 65 J = 1, NRHS * * Solve L**H * x = b. * DO 50 I = N - 1, 1, -1 B( I, J ) = B( I, J ) - $ B( I+1, J )*( E( I ) ) 50 CONTINUE 65 CONTINUE ENDIF * RETURN * * End of DPTTRS * END