LAPACK 3.3.0

spotrs.f

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00001       SUBROUTINE SPOTRS( UPLO, N, NRHS, A, LDA, B, LDB, INFO )
00002 *
00003 *  -- LAPACK routine (version 3.2) --
00004 *  -- LAPACK is a software package provided by Univ. of Tennessee,    --
00005 *  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
00006 *     November 2006
00007 *
00008 *     .. Scalar Arguments ..
00009       CHARACTER          UPLO
00010       INTEGER            INFO, LDA, LDB, N, NRHS
00011 *     ..
00012 *     .. Array Arguments ..
00013       REAL               A( LDA, * ), B( LDB, * )
00014 *     ..
00015 *
00016 *  Purpose
00017 *  =======
00018 *
00019 *  SPOTRS solves a system of linear equations A*X = B with a symmetric
00020 *  positive definite matrix A using the Cholesky factorization
00021 *  A = U**T*U or A = L*L**T computed by SPOTRF.
00022 *
00023 *  Arguments
00024 *  =========
00025 *
00026 *  UPLO    (input) CHARACTER*1
00027 *          = 'U':  Upper triangle of A is stored;
00028 *          = 'L':  Lower triangle of A is stored.
00029 *
00030 *  N       (input) INTEGER
00031 *          The order of the matrix A.  N >= 0.
00032 *
00033 *  NRHS    (input) INTEGER
00034 *          The number of right hand sides, i.e., the number of columns
00035 *          of the matrix B.  NRHS >= 0.
00036 *
00037 *  A       (input) REAL array, dimension (LDA,N)
00038 *          The triangular factor U or L from the Cholesky factorization
00039 *          A = U**T*U or A = L*L**T, as computed by SPOTRF.
00040 *
00041 *  LDA     (input) INTEGER
00042 *          The leading dimension of the array A.  LDA >= max(1,N).
00043 *
00044 *  B       (input/output) REAL array, dimension (LDB,NRHS)
00045 *          On entry, the right hand side matrix B.
00046 *          On exit, the solution matrix X.
00047 *
00048 *  LDB     (input) INTEGER
00049 *          The leading dimension of the array B.  LDB >= max(1,N).
00050 *
00051 *  INFO    (output) INTEGER
00052 *          = 0:  successful exit
00053 *          < 0:  if INFO = -i, the i-th argument had an illegal value
00054 *
00055 *  =====================================================================
00056 *
00057 *     .. Parameters ..
00058       REAL               ONE
00059       PARAMETER          ( ONE = 1.0E+0 )
00060 *     ..
00061 *     .. Local Scalars ..
00062       LOGICAL            UPPER
00063 *     ..
00064 *     .. External Functions ..
00065       LOGICAL            LSAME
00066       EXTERNAL           LSAME
00067 *     ..
00068 *     .. External Subroutines ..
00069       EXTERNAL           STRSM, XERBLA
00070 *     ..
00071 *     .. Intrinsic Functions ..
00072       INTRINSIC          MAX
00073 *     ..
00074 *     .. Executable Statements ..
00075 *
00076 *     Test the input parameters.
00077 *
00078       INFO = 0
00079       UPPER = LSAME( UPLO, 'U' )
00080       IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
00081          INFO = -1
00082       ELSE IF( N.LT.0 ) THEN
00083          INFO = -2
00084       ELSE IF( NRHS.LT.0 ) THEN
00085          INFO = -3
00086       ELSE IF( LDA.LT.MAX( 1, N ) ) THEN
00087          INFO = -5
00088       ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
00089          INFO = -7
00090       END IF
00091       IF( INFO.NE.0 ) THEN
00092          CALL XERBLA( 'SPOTRS', -INFO )
00093          RETURN
00094       END IF
00095 *
00096 *     Quick return if possible
00097 *
00098       IF( N.EQ.0 .OR. NRHS.EQ.0 )
00099      $   RETURN
00100 *
00101       IF( UPPER ) THEN
00102 *
00103 *        Solve A*X = B where A = U'*U.
00104 *
00105 *        Solve U'*X = B, overwriting B with X.
00106 *
00107          CALL STRSM( 'Left', 'Upper', 'Transpose', 'Non-unit', N, NRHS,
00108      $               ONE, A, LDA, B, LDB )
00109 *
00110 *        Solve U*X = B, overwriting B with X.
00111 *
00112          CALL STRSM( 'Left', 'Upper', 'No transpose', 'Non-unit', N,
00113      $               NRHS, ONE, A, LDA, B, LDB )
00114       ELSE
00115 *
00116 *        Solve A*X = B where A = L*L'.
00117 *
00118 *        Solve L*X = B, overwriting B with X.
00119 *
00120          CALL STRSM( 'Left', 'Lower', 'No transpose', 'Non-unit', N,
00121      $               NRHS, ONE, A, LDA, B, LDB )
00122 *
00123 *        Solve L'*X = B, overwriting B with X.
00124 *
00125          CALL STRSM( 'Left', 'Lower', 'Transpose', 'Non-unit', N, NRHS,
00126      $               ONE, A, LDA, B, LDB )
00127       END IF
00128 *
00129       RETURN
00130 *
00131 *     End of SPOTRS
00132 *
00133       END
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