LAPACK: BLAS/SRC/ctbsv.f Source File

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00001       SUBROUTINE CTBSV(UPLO,TRANS,DIAG,N,K,A,LDA,X,INCX)
00002 *     .. Scalar Arguments ..
00003       INTEGER INCX,K,LDA,N
00004       CHARACTER DIAG,TRANS,UPLO
00005 *     ..
00006 *     .. Array Arguments ..
00007       COMPLEX A(LDA,*),X(*)
00008 *     ..
00009 *
00010 *  Purpose
00011 *  =======
00012 *
00013 *  CTBSV  solves one of the systems of equations
00014 *
00015 *     A*x = b,   or   A'*x = b,   or   conjg( A' )*x = b,
00016 *
00017 *  where b and x are n element vectors and A is an n by n unit, or
00018 *  non-unit, upper or lower triangular band matrix, with ( k + 1 )
00019 *  diagonals.
00020 *
00021 *  No test for singularity or near-singularity is included in this
00022 *  routine. Such tests must be performed before calling this routine.
00023 *
00024 *  Arguments
00025 *  ==========
00026 *
00027 *  UPLO   - CHARACTER*1.
00028 *           On entry, UPLO specifies whether the matrix is an upper or
00029 *           lower triangular matrix as follows:
00030 *
00031 *              UPLO = 'U' or 'u'   A is an upper triangular matrix.
00032 *
00033 *              UPLO = 'L' or 'l'   A is a lower triangular matrix.
00034 *
00035 *           Unchanged on exit.
00036 *
00037 *  TRANS  - CHARACTER*1.
00038 *           On entry, TRANS specifies the equations to be solved as
00039 *           follows:
00040 *
00041 *              TRANS = 'N' or 'n'   A*x = b.
00042 *
00043 *              TRANS = 'T' or 't'   A'*x = b.
00044 *
00045 *              TRANS = 'C' or 'c'   conjg( A' )*x = b.
00046 *
00047 *           Unchanged on exit.
00048 *
00049 *  DIAG   - CHARACTER*1.
00050 *           On entry, DIAG specifies whether or not A is unit
00051 *           triangular as follows:
00052 *
00053 *              DIAG = 'U' or 'u'   A is assumed to be unit triangular.
00054 *
00055 *              DIAG = 'N' or 'n'   A is not assumed to be unit
00056 *                                  triangular.
00057 *
00058 *           Unchanged on exit.
00059 *
00060 *  N      - INTEGER.
00061 *           On entry, N specifies the order of the matrix A.
00062 *           N must be at least zero.
00063 *           Unchanged on exit.
00064 *
00065 *  K      - INTEGER.
00066 *           On entry with UPLO = 'U' or 'u', K specifies the number of
00067 *           super-diagonals of the matrix A.
00068 *           On entry with UPLO = 'L' or 'l', K specifies the number of
00069 *           sub-diagonals of the matrix A.
00070 *           K must satisfy  0 .le. K.
00071 *           Unchanged on exit.
00072 *
00073 *  A      - COMPLEX          array of DIMENSION ( LDA, n ).
00074 *           Before entry with UPLO = 'U' or 'u', the leading ( k + 1 )
00075 *           by n part of the array A must contain the upper triangular
00076 *           band part of the matrix of coefficients, supplied column by
00077 *           column, with the leading diagonal of the matrix in row
00078 *           ( k + 1 ) of the array, the first super-diagonal starting at
00079 *           position 2 in row k, and so on. The top left k by k triangle
00080 *           of the array A is not referenced.
00081 *           The following program segment will transfer an upper
00082 *           triangular band matrix from conventional full matrix storage
00083 *           to band storage:
00084 *
00085 *                 DO 20, J = 1, N
00086 *                    M = K + 1 - J
00087 *                    DO 10, I = MAX( 1, J - K ), J
00088 *                       A( M + I, J ) = matrix( I, J )
00089 *              10    CONTINUE
00090 *              20 CONTINUE
00091 *
00092 *           Before entry with UPLO = 'L' or 'l', the leading ( k + 1 )
00093 *           by n part of the array A must contain the lower triangular
00094 *           band part of the matrix of coefficients, supplied column by
00095 *           column, with the leading diagonal of the matrix in row 1 of
00096 *           the array, the first sub-diagonal starting at position 1 in
00097 *           row 2, and so on. The bottom right k by k triangle of the
00098 *           array A is not referenced.
00099 *           The following program segment will transfer a lower
00100 *           triangular band matrix from conventional full matrix storage
00101 *           to band storage:
00102 *
00103 *                 DO 20, J = 1, N
00104 *                    M = 1 - J
00105 *                    DO 10, I = J, MIN( N, J + K )
00106 *                       A( M + I, J ) = matrix( I, J )
00107 *              10    CONTINUE
00108 *              20 CONTINUE
00109 *
00110 *           Note that when DIAG = 'U' or 'u' the elements of the array A
00111 *           corresponding to the diagonal elements of the matrix are not
00112 *           referenced, but are assumed to be unity.
00113 *           Unchanged on exit.
00114 *
00115 *  LDA    - INTEGER.
00116 *           On entry, LDA specifies the first dimension of A as declared
00117 *           in the calling (sub) program. LDA must be at least
00118 *           ( k + 1 ).
00119 *           Unchanged on exit.
00120 *
00121 *  X      - COMPLEX          array of dimension at least
00122 *           ( 1 + ( n - 1 )*abs( INCX ) ).
00123 *           Before entry, the incremented array X must contain the n
00124 *           element right-hand side vector b. On exit, X is overwritten
00125 *           with the solution vector x.
00126 *
00127 *  INCX   - INTEGER.
00128 *           On entry, INCX specifies the increment for the elements of
00129 *           X. INCX must not be zero.
00130 *           Unchanged on exit.
00131 *
00132 *  Further Details
00133 *  ===============
00134 *
00135 *  Level 2 Blas routine.
00136 *
00137 *  -- Written on 22-October-1986.
00138 *     Jack Dongarra, Argonne National Lab.
00139 *     Jeremy Du Croz, Nag Central Office.
00140 *     Sven Hammarling, Nag Central Office.
00141 *     Richard Hanson, Sandia National Labs.
00142 *
00143 *  =====================================================================
00144 *
00145 *     .. Parameters ..
00146       COMPLEX ZERO
00147       PARAMETER (ZERO= (0.0E+0,0.0E+0))
00148 *     ..
00149 *     .. Local Scalars ..
00150       COMPLEX TEMP
00151       INTEGER I,INFO,IX,J,JX,KPLUS1,KX,L
00152       LOGICAL NOCONJ,NOUNIT
00153 *     ..
00154 *     .. External Functions ..
00155       LOGICAL LSAME
00156       EXTERNAL LSAME
00157 *     ..
00158 *     .. External Subroutines ..
00159       EXTERNAL XERBLA
00160 *     ..
00161 *     .. Intrinsic Functions ..
00162       INTRINSIC CONJG,MAX,MIN
00163 *     ..
00164 *
00165 *     Test the input parameters.
00166 *
00167       INFO = 0
00168       IF (.NOT.LSAME(UPLO,'U') .AND. .NOT.LSAME(UPLO,'L')) THEN
00169           INFO = 1
00170       ELSE IF (.NOT.LSAME(TRANS,'N') .AND. .NOT.LSAME(TRANS,'T') .AND.
00171      +         .NOT.LSAME(TRANS,'C')) THEN
00172           INFO = 2
00173       ELSE IF (.NOT.LSAME(DIAG,'U') .AND. .NOT.LSAME(DIAG,'N')) THEN
00174           INFO = 3
00175       ELSE IF (N.LT.0) THEN
00176           INFO = 4
00177       ELSE IF (K.LT.0) THEN
00178           INFO = 5
00179       ELSE IF (LDA.LT. (K+1)) THEN
00180           INFO = 7
00181       ELSE IF (INCX.EQ.0) THEN
00182           INFO = 9
00183       END IF
00184       IF (INFO.NE.0) THEN
00185           CALL XERBLA('CTBSV ',INFO)
00186           RETURN
00187       END IF
00188 *
00189 *     Quick return if possible.
00190 *
00191       IF (N.EQ.0) RETURN
00192 *
00193       NOCONJ = LSAME(TRANS,'T')
00194       NOUNIT = LSAME(DIAG,'N')
00195 *
00196 *     Set up the start point in X if the increment is not unity. This
00197 *     will be  ( N - 1 )*INCX  too small for descending loops.
00198 *
00199       IF (INCX.LE.0) THEN
00200           KX = 1 - (N-1)*INCX
00201       ELSE IF (INCX.NE.1) THEN
00202           KX = 1
00203       END IF
00204 *
00205 *     Start the operations. In this version the elements of A are
00206 *     accessed by sequentially with one pass through A.
00207 *
00208       IF (LSAME(TRANS,'N')) THEN
00209 *
00210 *        Form  x := inv( A )*x.
00211 *
00212           IF (LSAME(UPLO,'U')) THEN
00213               KPLUS1 = K + 1
00214               IF (INCX.EQ.1) THEN
00215                   DO 20 J = N,1,-1
00216                       IF (X(J).NE.ZERO) THEN
00217                           L = KPLUS1 - J
00218                           IF (NOUNIT) X(J) = X(J)/A(KPLUS1,J)
00219                           TEMP = X(J)
00220                           DO 10 I = J - 1,MAX(1,J-K),-1
00221                               X(I) = X(I) - TEMP*A(L+I,J)
00222    10                     CONTINUE
00223                       END IF
00224    20             CONTINUE
00225               ELSE
00226                   KX = KX + (N-1)*INCX
00227                   JX = KX
00228                   DO 40 J = N,1,-1
00229                       KX = KX - INCX
00230                       IF (X(JX).NE.ZERO) THEN
00231                           IX = KX
00232                           L = KPLUS1 - J
00233                           IF (NOUNIT) X(JX) = X(JX)/A(KPLUS1,J)
00234                           TEMP = X(JX)
00235                           DO 30 I = J - 1,MAX(1,J-K),-1
00236                               X(IX) = X(IX) - TEMP*A(L+I,J)
00237                               IX = IX - INCX
00238    30                     CONTINUE
00239                       END IF
00240                       JX = JX - INCX
00241    40             CONTINUE
00242               END IF
00243           ELSE
00244               IF (INCX.EQ.1) THEN
00245                   DO 60 J = 1,N
00246                       IF (X(J).NE.ZERO) THEN
00247                           L = 1 - J
00248                           IF (NOUNIT) X(J) = X(J)/A(1,J)
00249                           TEMP = X(J)
00250                           DO 50 I = J + 1,MIN(N,J+K)
00251                               X(I) = X(I) - TEMP*A(L+I,J)
00252    50                     CONTINUE
00253                       END IF
00254    60             CONTINUE
00255               ELSE
00256                   JX = KX
00257                   DO 80 J = 1,N
00258                       KX = KX + INCX
00259                       IF (X(JX).NE.ZERO) THEN
00260                           IX = KX
00261                           L = 1 - J
00262                           IF (NOUNIT) X(JX) = X(JX)/A(1,J)
00263                           TEMP = X(JX)
00264                           DO 70 I = J + 1,MIN(N,J+K)
00265                               X(IX) = X(IX) - TEMP*A(L+I,J)
00266                               IX = IX + INCX
00267    70                     CONTINUE
00268                       END IF
00269                       JX = JX + INCX
00270    80             CONTINUE
00271               END IF
00272           END IF
00273       ELSE
00274 *
00275 *        Form  x := inv( A' )*x  or  x := inv( conjg( A') )*x.
00276 *
00277           IF (LSAME(UPLO,'U')) THEN
00278               KPLUS1 = K + 1
00279               IF (INCX.EQ.1) THEN
00280                   DO 110 J = 1,N
00281                       TEMP = X(J)
00282                       L = KPLUS1 - J
00283                       IF (NOCONJ) THEN
00284                           DO 90 I = MAX(1,J-K),J - 1
00285                               TEMP = TEMP - A(L+I,J)*X(I)
00286    90                     CONTINUE
00287                           IF (NOUNIT) TEMP = TEMP/A(KPLUS1,J)
00288                       ELSE
00289                           DO 100 I = MAX(1,J-K),J - 1
00290                               TEMP = TEMP - CONJG(A(L+I,J))*X(I)
00291   100                     CONTINUE
00292                           IF (NOUNIT) TEMP = TEMP/CONJG(A(KPLUS1,J))
00293                       END IF
00294                       X(J) = TEMP
00295   110             CONTINUE
00296               ELSE
00297                   JX = KX
00298                   DO 140 J = 1,N
00299                       TEMP = X(JX)
00300                       IX = KX
00301                       L = KPLUS1 - J
00302                       IF (NOCONJ) THEN
00303                           DO 120 I = MAX(1,J-K),J - 1
00304                               TEMP = TEMP - A(L+I,J)*X(IX)
00305                               IX = IX + INCX
00306   120                     CONTINUE
00307                           IF (NOUNIT) TEMP = TEMP/A(KPLUS1,J)
00308                       ELSE
00309                           DO 130 I = MAX(1,J-K),J - 1
00310                               TEMP = TEMP - CONJG(A(L+I,J))*X(IX)
00311                               IX = IX + INCX
00312   130                     CONTINUE
00313                           IF (NOUNIT) TEMP = TEMP/CONJG(A(KPLUS1,J))
00314                       END IF
00315                       X(JX) = TEMP
00316                       JX = JX + INCX
00317                       IF (J.GT.K) KX = KX + INCX
00318   140             CONTINUE
00319               END IF
00320           ELSE
00321               IF (INCX.EQ.1) THEN
00322                   DO 170 J = N,1,-1
00323                       TEMP = X(J)
00324                       L = 1 - J
00325                       IF (NOCONJ) THEN
00326                           DO 150 I = MIN(N,J+K),J + 1,-1
00327                               TEMP = TEMP - A(L+I,J)*X(I)
00328   150                     CONTINUE
00329                           IF (NOUNIT) TEMP = TEMP/A(1,J)
00330                       ELSE
00331                           DO 160 I = MIN(N,J+K),J + 1,-1
00332                               TEMP = TEMP - CONJG(A(L+I,J))*X(I)
00333   160                     CONTINUE
00334                           IF (NOUNIT) TEMP = TEMP/CONJG(A(1,J))
00335                       END IF
00336                       X(J) = TEMP
00337   170             CONTINUE
00338               ELSE
00339                   KX = KX + (N-1)*INCX
00340                   JX = KX
00341                   DO 200 J = N,1,-1
00342                       TEMP = X(JX)
00343                       IX = KX
00344                       L = 1 - J
00345                       IF (NOCONJ) THEN
00346                           DO 180 I = MIN(N,J+K),J + 1,-1
00347                               TEMP = TEMP - A(L+I,J)*X(IX)
00348                               IX = IX - INCX
00349   180                     CONTINUE
00350                           IF (NOUNIT) TEMP = TEMP/A(1,J)
00351                       ELSE
00352                           DO 190 I = MIN(N,J+K),J + 1,-1
00353                               TEMP = TEMP - CONJG(A(L+I,J))*X(IX)
00354                               IX = IX - INCX
00355   190                     CONTINUE
00356                           IF (NOUNIT) TEMP = TEMP/CONJG(A(1,J))
00357                       END IF
00358                       X(JX) = TEMP
00359                       JX = JX - INCX
00360                       IF ((N-J).GE.K) KX = KX - INCX
00361   200             CONTINUE
00362               END IF
00363           END IF
00364       END IF
00365 *
00366       RETURN
00367 *
00368 *     End of CTBSV .
00369 *
00370       END