SUBROUTINE SPTTRS( N, NRHS, D, E, B, LDB, INFO ) * * -- LAPACK routine (version 3.2) -- * Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd.. * November 2006 * * .. Scalar Arguments .. INTEGER INFO, LDB, N, NRHS * .. * .. Array Arguments .. REAL B( LDB, * ), D( * ), E( * ) * .. * * Purpose * ======= * * SPTTRS solves a tridiagonal system of the form * A * X = B * using the L*D*L' factorization of A computed by SPTTRF. D is a * diagonal matrix specified in the vector D, L is a unit bidiagonal * matrix whose subdiagonal is specified in the vector E, and X and B * are N by NRHS matrices. * * Arguments * ========= * * 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 * L*D*L' factorization of A. * * E (input) REAL array, dimension (N-1) * The (n-1) subdiagonal elements of the unit bidiagonal factor * L from the L*D*L' factorization of A. E can also be regarded * as the superdiagonal of the unit bidiagonal factor U from the * factorization A = U'*D*U. * * B (input/output) REAL 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 .. INTEGER J, JB, NB * .. * .. External Functions .. INTEGER ILAENV EXTERNAL ILAENV * .. * .. External Subroutines .. EXTERNAL SPTTS2, XERBLA * .. * .. Intrinsic Functions .. INTRINSIC MAX, MIN * .. * .. Executable Statements .. * * Test the input arguments. * INFO = 0 IF( N.LT.0 ) THEN INFO = -1 ELSE IF( NRHS.LT.0 ) THEN INFO = -2 ELSE IF( LDB.LT.MAX( 1, N ) ) THEN INFO = -6 END IF IF( INFO.NE.0 ) THEN CALL XERBLA( 'SPTTRS', -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, 'SPTTRS', ' ', N, NRHS, -1, -1 ) ) END IF * IF( NB.GE.NRHS ) THEN CALL SPTTS2( N, NRHS, D, E, B, LDB ) ELSE DO 10 J = 1, NRHS, NB JB = MIN( NRHS-J+1, NB ) CALL SPTTS2( N, JB, D, E, B( 1, J ), LDB ) 10 CONTINUE END IF * RETURN * * End of SPTTRS * END