SUBROUTINE ZPTTRS( UPLO, N, NRHS, D, E, B, LDB, INFO ) * * -- LAPACK routine (version 3.2) -- * -- LAPACK is a software package provided by Univ. of Tennessee, -- * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..-- * November 2006 * * .. Scalar Arguments .. CHARACTER UPLO INTEGER INFO, LDB, N, NRHS * .. * .. Array Arguments .. DOUBLE PRECISION D( * ) COMPLEX*16 B( LDB, * ), E( * ) * .. * * Purpose * ======= * * ZPTTRS solves a tridiagonal system of the form * A * X = B * using the factorization A = U'*D*U or A = L*D*L' computed by ZPTTRF. * D is a diagonal matrix specified in the vector D, U (or L) is a unit * bidiagonal matrix whose superdiagonal (subdiagonal) is specified in * the vector E, and X and B are N by NRHS matrices. * * Arguments * ========= * * UPLO (input) CHARACTER*1 * Specifies the form of the factorization and whether the * vector E is the superdiagonal of the upper bidiagonal factor * U or the subdiagonal of the lower bidiagonal factor L. * = 'U': A = U'*D*U, E is the superdiagonal of U * = 'L': A = L*D*L', E is the subdiagonal of L * * 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) DOUBLE PRECISION array, dimension (N) * The n diagonal elements of the diagonal matrix D from the * factorization A = U'*D*U or A = L*D*L'. * * E (input) COMPLEX*16 array, dimension (N-1) * If UPLO = 'U', the (n-1) superdiagonal elements of the unit * bidiagonal factor U from the factorization A = U'*D*U. * If UPLO = 'L', the (n-1) subdiagonal elements of the unit * bidiagonal factor L from the factorization A = L*D*L'. * * B (input/output) DOUBLE PRECISION array, dimension (LDB,NRHS) * On entry, the right hand side vectors B for the system of * linear equations. * On exit, the solution vectors, X. * * LDB (input) INTEGER * The leading dimension of the array B. LDB >= max(1,N). * * INFO (output) INTEGER * = 0: successful exit * < 0: if INFO = -k, the k-th argument had an illegal value * * ===================================================================== * * .. Local Scalars .. LOGICAL UPPER INTEGER IUPLO, J, JB, NB * .. * .. External Functions .. INTEGER ILAENV EXTERNAL ILAENV * .. * .. External Subroutines .. EXTERNAL XERBLA, ZPTTS2 * .. * .. Intrinsic Functions .. INTRINSIC MAX, MIN * .. * .. Executable Statements .. * * Test the input arguments. * INFO = 0 UPPER = ( UPLO.EQ.'U' .OR. UPLO.EQ.'u' ) IF( .NOT.UPPER .AND. .NOT.( UPLO.EQ.'L' .OR. UPLO.EQ.'l' ) ) 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( 'ZPTTRS', -INFO ) RETURN END IF * * Quick return if possible * IF( N.EQ.0 .OR. NRHS.EQ.0 ) \$ RETURN * * Determine the number of right-hand sides to solve at a time. * IF( NRHS.EQ.1 ) THEN NB = 1 ELSE NB = MAX( 1, ILAENV( 1, 'ZPTTRS', UPLO, N, NRHS, -1, -1 ) ) END IF * * Decode UPLO * IF( UPPER ) THEN IUPLO = 1 ELSE IUPLO = 0 END IF * IF( NB.GE.NRHS ) THEN CALL ZPTTS2( IUPLO, N, NRHS, D, E, B, LDB ) ELSE DO 10 J = 1, NRHS, NB JB = MIN( NRHS-J+1, NB ) CALL ZPTTS2( IUPLO, N, JB, D, E, B( 1, J ), LDB ) 10 CONTINUE END IF * RETURN * * End of ZPTTRS * END