LAPACK  3.4.1
LAPACK: Linear Algebra PACKage
dsysv.f
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00001 *> \brief <b> DSYSV computes the solution to system of linear equations A * X = B for SY matrices</b>
00002 *
00003 *  =========== DOCUMENTATION ===========
00004 *
00005 * Online html documentation available at 
00006 *            http://www.netlib.org/lapack/explore-html/ 
00007 *
00008 *> \htmlonly
00009 *> Download DSYSV + dependencies 
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00011 *> [TGZ]</a> 
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00013 *> [ZIP]</a> 
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00015 *> [TXT]</a>
00016 *> \endhtmlonly 
00017 *
00018 *  Definition:
00019 *  ===========
00020 *
00021 *       SUBROUTINE DSYSV( UPLO, N, NRHS, A, LDA, IPIV, B, LDB, WORK,
00022 *                         LWORK, INFO )
00023 * 
00024 *       .. Scalar Arguments ..
00025 *       CHARACTER          UPLO
00026 *       INTEGER            INFO, LDA, LDB, LWORK, N, NRHS
00027 *       ..
00028 *       .. Array Arguments ..
00029 *       INTEGER            IPIV( * )
00030 *       DOUBLE PRECISION   A( LDA, * ), B( LDB, * ), WORK( * )
00031 *       ..
00032 *  
00033 *
00034 *> \par Purpose:
00035 *  =============
00036 *>
00037 *> \verbatim
00038 *>
00039 *> DSYSV computes the solution to a real system of linear equations
00040 *>    A * X = B,
00041 *> where A is an N-by-N symmetric matrix and X and B are N-by-NRHS
00042 *> matrices.
00043 *>
00044 *> The diagonal pivoting method is used to factor A as
00045 *>    A = U * D * U**T,  if UPLO = 'U', or
00046 *>    A = L * D * L**T,  if UPLO = 'L',
00047 *> where U (or L) is a product of permutation and unit upper (lower)
00048 *> triangular matrices, and D is symmetric and block diagonal with
00049 *> 1-by-1 and 2-by-2 diagonal blocks.  The factored form of A is then
00050 *> used to solve the system of equations A * X = B.
00051 *> \endverbatim
00052 *
00053 *  Arguments:
00054 *  ==========
00055 *
00056 *> \param[in] UPLO
00057 *> \verbatim
00058 *>          UPLO is CHARACTER*1
00059 *>          = 'U':  Upper triangle of A is stored;
00060 *>          = 'L':  Lower triangle of A is stored.
00061 *> \endverbatim
00062 *>
00063 *> \param[in] N
00064 *> \verbatim
00065 *>          N is INTEGER
00066 *>          The number of linear equations, i.e., the order of the
00067 *>          matrix A.  N >= 0.
00068 *> \endverbatim
00069 *>
00070 *> \param[in] NRHS
00071 *> \verbatim
00072 *>          NRHS is INTEGER
00073 *>          The number of right hand sides, i.e., the number of columns
00074 *>          of the matrix B.  NRHS >= 0.
00075 *> \endverbatim
00076 *>
00077 *> \param[in,out] A
00078 *> \verbatim
00079 *>          A is DOUBLE PRECISION array, dimension (LDA,N)
00080 *>          On entry, the symmetric matrix A.  If UPLO = 'U', the leading
00081 *>          N-by-N upper triangular part of A contains the upper
00082 *>          triangular part of the matrix A, and the strictly lower
00083 *>          triangular part of A is not referenced.  If UPLO = 'L', the
00084 *>          leading N-by-N lower triangular part of A contains the lower
00085 *>          triangular part of the matrix A, and the strictly upper
00086 *>          triangular part of A is not referenced.
00087 *>
00088 *>          On exit, if INFO = 0, the block diagonal matrix D and the
00089 *>          multipliers used to obtain the factor U or L from the
00090 *>          factorization A = U*D*U**T or A = L*D*L**T as computed by
00091 *>          DSYTRF.
00092 *> \endverbatim
00093 *>
00094 *> \param[in] LDA
00095 *> \verbatim
00096 *>          LDA is INTEGER
00097 *>          The leading dimension of the array A.  LDA >= max(1,N).
00098 *> \endverbatim
00099 *>
00100 *> \param[out] IPIV
00101 *> \verbatim
00102 *>          IPIV is INTEGER array, dimension (N)
00103 *>          Details of the interchanges and the block structure of D, as
00104 *>          determined by DSYTRF.  If IPIV(k) > 0, then rows and columns
00105 *>          k and IPIV(k) were interchanged, and D(k,k) is a 1-by-1
00106 *>          diagonal block.  If UPLO = 'U' and IPIV(k) = IPIV(k-1) < 0,
00107 *>          then rows and columns k-1 and -IPIV(k) were interchanged and
00108 *>          D(k-1:k,k-1:k) is a 2-by-2 diagonal block.  If UPLO = 'L' and
00109 *>          IPIV(k) = IPIV(k+1) < 0, then rows and columns k+1 and
00110 *>          -IPIV(k) were interchanged and D(k:k+1,k:k+1) is a 2-by-2
00111 *>          diagonal block.
00112 *> \endverbatim
00113 *>
00114 *> \param[in,out] B
00115 *> \verbatim
00116 *>          B is DOUBLE PRECISION array, dimension (LDB,NRHS)
00117 *>          On entry, the N-by-NRHS right hand side matrix B.
00118 *>          On exit, if INFO = 0, the N-by-NRHS solution matrix X.
00119 *> \endverbatim
00120 *>
00121 *> \param[in] LDB
00122 *> \verbatim
00123 *>          LDB is INTEGER
00124 *>          The leading dimension of the array B.  LDB >= max(1,N).
00125 *> \endverbatim
00126 *>
00127 *> \param[out] WORK
00128 *> \verbatim
00129 *>          WORK is DOUBLE PRECISION array, dimension (MAX(1,LWORK))
00130 *>          On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
00131 *> \endverbatim
00132 *>
00133 *> \param[in] LWORK
00134 *> \verbatim
00135 *>          LWORK is INTEGER
00136 *>          The length of WORK.  LWORK >= 1, and for best performance
00137 *>          LWORK >= max(1,N*NB), where NB is the optimal blocksize for
00138 *>          DSYTRF.
00139 *>          for LWORK < N, TRS will be done with Level BLAS 2
00140 *>          for LWORK >= N, TRS will be done with Level BLAS 3
00141 *>
00142 *>          If LWORK = -1, then a workspace query is assumed; the routine
00143 *>          only calculates the optimal size of the WORK array, returns
00144 *>          this value as the first entry of the WORK array, and no error
00145 *>          message related to LWORK is issued by XERBLA.
00146 *> \endverbatim
00147 *>
00148 *> \param[out] INFO
00149 *> \verbatim
00150 *>          INFO is INTEGER
00151 *>          = 0: successful exit
00152 *>          < 0: if INFO = -i, the i-th argument had an illegal value
00153 *>          > 0: if INFO = i, D(i,i) is exactly zero.  The factorization
00154 *>               has been completed, but the block diagonal matrix D is
00155 *>               exactly singular, so the solution could not be computed.
00156 *> \endverbatim
00157 *
00158 *  Authors:
00159 *  ========
00160 *
00161 *> \author Univ. of Tennessee 
00162 *> \author Univ. of California Berkeley 
00163 *> \author Univ. of Colorado Denver 
00164 *> \author NAG Ltd. 
00165 *
00166 *> \date November 2011
00167 *
00168 *> \ingroup doubleSYsolve
00169 *
00170 *  =====================================================================
00171       SUBROUTINE DSYSV( UPLO, N, NRHS, A, LDA, IPIV, B, LDB, WORK,
00172      $                  LWORK, INFO )
00173 *
00174 *  -- LAPACK driver routine (version 3.4.0) --
00175 *  -- LAPACK is a software package provided by Univ. of Tennessee,    --
00176 *  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
00177 *     November 2011
00178 *
00179 *     .. Scalar Arguments ..
00180       CHARACTER          UPLO
00181       INTEGER            INFO, LDA, LDB, LWORK, N, NRHS
00182 *     ..
00183 *     .. Array Arguments ..
00184       INTEGER            IPIV( * )
00185       DOUBLE PRECISION   A( LDA, * ), B( LDB, * ), WORK( * )
00186 *     ..
00187 *
00188 *  =====================================================================
00189 *
00190 *     .. Local Scalars ..
00191       LOGICAL            LQUERY
00192       INTEGER            LWKOPT
00193 *     ..
00194 *     .. External Functions ..
00195       LOGICAL            LSAME
00196       EXTERNAL           LSAME
00197 *     ..
00198 *     .. External Subroutines ..
00199       EXTERNAL           XERBLA, DSYTRF, DSYTRS, DSYTRS2
00200 *     ..
00201 *     .. Intrinsic Functions ..
00202       INTRINSIC          MAX
00203 *     ..
00204 *     .. Executable Statements ..
00205 *
00206 *     Test the input parameters.
00207 *
00208       INFO = 0
00209       LQUERY = ( LWORK.EQ.-1 )
00210       IF( .NOT.LSAME( UPLO, 'U' ) .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
00211          INFO = -1
00212       ELSE IF( N.LT.0 ) THEN
00213          INFO = -2
00214       ELSE IF( NRHS.LT.0 ) THEN
00215          INFO = -3
00216       ELSE IF( LDA.LT.MAX( 1, N ) ) THEN
00217          INFO = -5
00218       ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
00219          INFO = -8
00220       ELSE IF( LWORK.LT.1 .AND. .NOT.LQUERY ) THEN
00221          INFO = -10
00222       END IF
00223 *
00224       IF( INFO.EQ.0 ) THEN
00225          IF( N.EQ.0 ) THEN
00226             LWKOPT = 1
00227          ELSE
00228             CALL DSYTRF( UPLO, N, A, LDA, IPIV, WORK, -1, INFO )
00229             LWKOPT = WORK(1)
00230          END IF
00231          WORK( 1 ) = LWKOPT
00232       END IF
00233 *
00234       IF( INFO.NE.0 ) THEN
00235          CALL XERBLA( 'DSYSV ', -INFO )
00236          RETURN
00237       ELSE IF( LQUERY ) THEN
00238          RETURN
00239       END IF
00240 *
00241 *     Compute the factorization A = U*D*U**T or A = L*D*L**T.
00242 *
00243       CALL DSYTRF( UPLO, N, A, LDA, IPIV, WORK, LWORK, INFO )
00244       IF( INFO.EQ.0 ) THEN
00245 *
00246 *        Solve the system A*X = B, overwriting B with X.
00247 *
00248          IF ( LWORK.LT.N ) THEN
00249 *
00250 *        Solve with TRS ( Use Level BLAS 2)
00251 *
00252             CALL DSYTRS( UPLO, N, NRHS, A, LDA, IPIV, B, LDB, INFO )
00253 *
00254          ELSE
00255 *
00256 *        Solve with TRS2 ( Use Level BLAS 3)
00257 *
00258             CALL DSYTRS2( UPLO,N,NRHS,A,LDA,IPIV,B,LDB,WORK,INFO )
00259 *
00260          END IF
00261 *
00262       END IF
00263 *
00264       WORK( 1 ) = LWKOPT
00265 *
00266       RETURN
00267 *
00268 *     End of DSYSV
00269 *
00270       END
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