LAPACK 3.3.1
Linear Algebra PACKage

stptrs.f

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00001       SUBROUTINE STPTRS( UPLO, TRANS, DIAG, N, NRHS, AP, 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          DIAG, TRANS, UPLO
00010       INTEGER            INFO, LDB, N, NRHS
00011 *     ..
00012 *     .. Array Arguments ..
00013       REAL               AP( * ), B( LDB, * )
00014 *     ..
00015 *
00016 *  Purpose
00017 *  =======
00018 *
00019 *  STPTRS solves a triangular system of the form
00020 *
00021 *     A * X = B  or  A**T * X = B,
00022 *
00023 *  where A is a triangular matrix of order N stored in packed format,
00024 *  and B is an N-by-NRHS matrix.  A check is made to verify that A is
00025 *  nonsingular.
00026 *
00027 *  Arguments
00028 *  =========
00029 *
00030 *  UPLO    (input) CHARACTER*1
00031 *          = 'U':  A is upper triangular;
00032 *          = 'L':  A is lower triangular.
00033 *
00034 *  TRANS   (input) CHARACTER*1
00035 *          Specifies the form of the system of equations:
00036 *          = 'N':  A * X = B  (No transpose)
00037 *          = 'T':  A**T * X = B  (Transpose)
00038 *          = 'C':  A**H * X = B  (Conjugate transpose = Transpose)
00039 *
00040 *  DIAG    (input) CHARACTER*1
00041 *          = 'N':  A is non-unit triangular;
00042 *          = 'U':  A is unit triangular.
00043 *
00044 *  N       (input) INTEGER
00045 *          The order of the matrix A.  N >= 0.
00046 *
00047 *  NRHS    (input) INTEGER
00048 *          The number of right hand sides, i.e., the number of columns
00049 *          of the matrix B.  NRHS >= 0.
00050 *
00051 *  AP      (input) REAL array, dimension (N*(N+1)/2)
00052 *          The upper or lower triangular matrix A, packed columnwise in
00053 *          a linear array.  The j-th column of A is stored in the array
00054 *          AP as follows:
00055 *          if UPLO = 'U', AP(i + (j-1)*j/2) = A(i,j) for 1<=i<=j;
00056 *          if UPLO = 'L', AP(i + (j-1)*(2*n-j)/2) = A(i,j) for j<=i<=n.
00057 *
00058 *  B       (input/output) REAL array, dimension (LDB,NRHS)
00059 *          On entry, the right hand side matrix B.
00060 *          On exit, if INFO = 0, the solution matrix X.
00061 *
00062 *  LDB     (input) INTEGER
00063 *          The leading dimension of the array B.  LDB >= max(1,N).
00064 *
00065 *  INFO    (output) INTEGER
00066 *          = 0:  successful exit
00067 *          < 0:  if INFO = -i, the i-th argument had an illegal value
00068 *          > 0:  if INFO = i, the i-th diagonal element of A is zero,
00069 *                indicating that the matrix is singular and the
00070 *                solutions X have not been computed.
00071 *
00072 *  =====================================================================
00073 *
00074 *     .. Parameters ..
00075       REAL               ZERO
00076       PARAMETER          ( ZERO = 0.0E+0 )
00077 *     ..
00078 *     .. Local Scalars ..
00079       LOGICAL            NOUNIT, UPPER
00080       INTEGER            J, JC
00081 *     ..
00082 *     .. External Functions ..
00083       LOGICAL            LSAME
00084       EXTERNAL           LSAME
00085 *     ..
00086 *     .. External Subroutines ..
00087       EXTERNAL           STPSV, XERBLA
00088 *     ..
00089 *     .. Intrinsic Functions ..
00090       INTRINSIC          MAX
00091 *     ..
00092 *     .. Executable Statements ..
00093 *
00094 *     Test the input parameters.
00095 *
00096       INFO = 0
00097       UPPER = LSAME( UPLO, 'U' )
00098       NOUNIT = LSAME( DIAG, 'N' )
00099       IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
00100          INFO = -1
00101       ELSE IF( .NOT.LSAME( TRANS, 'N' ) .AND. .NOT.
00102      $         LSAME( TRANS, 'T' ) .AND. .NOT.LSAME( TRANS, 'C' ) ) THEN
00103          INFO = -2
00104       ELSE IF( .NOT.NOUNIT .AND. .NOT.LSAME( DIAG, 'U' ) ) THEN
00105          INFO = -3
00106       ELSE IF( N.LT.0 ) THEN
00107          INFO = -4
00108       ELSE IF( NRHS.LT.0 ) THEN
00109          INFO = -5
00110       ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
00111          INFO = -8
00112       END IF
00113       IF( INFO.NE.0 ) THEN
00114          CALL XERBLA( 'STPTRS', -INFO )
00115          RETURN
00116       END IF
00117 *
00118 *     Quick return if possible
00119 *
00120       IF( N.EQ.0 )
00121      $   RETURN
00122 *
00123 *     Check for singularity.
00124 *
00125       IF( NOUNIT ) THEN
00126          IF( UPPER ) THEN
00127             JC = 1
00128             DO 10 INFO = 1, N
00129                IF( AP( JC+INFO-1 ).EQ.ZERO )
00130      $            RETURN
00131                JC = JC + INFO
00132    10       CONTINUE
00133          ELSE
00134             JC = 1
00135             DO 20 INFO = 1, N
00136                IF( AP( JC ).EQ.ZERO )
00137      $            RETURN
00138                JC = JC + N - INFO + 1
00139    20       CONTINUE
00140          END IF
00141       END IF
00142       INFO = 0
00143 *
00144 *     Solve A * x = b  or  A**T * x = b.
00145 *
00146       DO 30 J = 1, NRHS
00147          CALL STPSV( UPLO, TRANS, DIAG, N, AP, B( 1, J ), 1 )
00148    30 CONTINUE
00149 *
00150       RETURN
00151 *
00152 *     End of STPTRS
00153 *
00154       END
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