SUBROUTINE DPPTRS( UPLO, N, NRHS, AP, 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 AP( * ), B( LDB, * ) * .. * * Purpose * ======= * * DPPTRS solves a system of linear equations A*X = B with a symmetric * positive definite matrix A in packed storage using the Cholesky * factorization A = U**T*U or A = L*L**T computed by DPPTRF. * * Arguments * ========= * * UPLO (input) CHARACTER*1 * = 'U': Upper triangle of A is stored; * = 'L': Lower triangle of A is stored. * * 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. * * AP (input) DOUBLE PRECISION array, dimension (N*(N+1)/2) * The triangular factor U or L from the Cholesky factorization * A = U**T*U or A = L*L**T, packed columnwise in a linear * array. The j-th column of U or L is stored in the array AP * as follows: * if UPLO = 'U', AP(i + (j-1)*j/2) = U(i,j) for 1<=i<=j; * if UPLO = 'L', AP(i + (j-1)*(2n-j)/2) = L(i,j) for j<=i<=n. * * 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 * * ===================================================================== * * .. Local Scalars .. LOGICAL UPPER INTEGER I * .. * .. External Functions .. LOGICAL LSAME EXTERNAL LSAME * .. * .. External Subroutines .. EXTERNAL DTPSV, XERBLA * .. * .. Intrinsic Functions .. INTRINSIC MAX * .. * .. Executable Statements .. * * Test the input parameters. * 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( LDB.LT.MAX( 1, N ) ) THEN INFO = -6 END IF IF( INFO.NE.0 ) THEN CALL XERBLA( 'DPPTRS', -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'*U. * DO 10 I = 1, NRHS * * Solve U'*X = B, overwriting B with X. * CALL DTPSV( 'Upper', 'Transpose', 'Non-unit', N, AP, \$ B( 1, I ), 1 ) * * Solve U*X = B, overwriting B with X. * CALL DTPSV( 'Upper', 'No transpose', 'Non-unit', N, AP, \$ B( 1, I ), 1 ) 10 CONTINUE ELSE * * Solve A*X = B where A = L*L'. * DO 20 I = 1, NRHS * * Solve L*Y = B, overwriting B with X. * CALL DTPSV( 'Lower', 'No transpose', 'Non-unit', N, AP, \$ B( 1, I ), 1 ) * * Solve L'*X = Y, overwriting B with X. * CALL DTPSV( 'Lower', 'Transpose', 'Non-unit', N, AP, \$ B( 1, I ), 1 ) 20 CONTINUE END IF * RETURN * * End of DPPTRS * END