SUBROUTINE DLARFB( SIDE, TRANS, DIRECT, STOREV, M, N, K, V, LDV,
     $                   T, LDT, C, LDC, WORK, LDWORK )
*
*  -- LAPACK auxiliary routine (version 3.1) --
*     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
*     November 2006
*
*     .. Scalar Arguments ..
      CHARACTER          DIRECT, SIDE, STOREV, TRANS
      INTEGER            K, LDC, LDT, LDV, LDWORK, M, N
*     ..
*     .. Array Arguments ..
      DOUBLE PRECISION   C( LDC, * ), T( LDT, * ), V( LDV, * ),
     $                   WORK( LDWORK, * )
*     ..
*
*  Purpose
*  =======
*
*  DLARFB applies a real block reflector H or its transpose H' to a
*  real m by n matrix C, from either the left or the right.
*
*  Arguments
*  =========
*
*  SIDE    (input) CHARACTER*1
*          = 'L': apply H or H' from the Left
*          = 'R': apply H or H' from the Right
*
*  TRANS   (input) CHARACTER*1
*          = 'N': apply H (No transpose)
*          = 'T': apply H' (Transpose)
*
*  DIRECT  (input) CHARACTER*1
*          Indicates how H is formed from a product of elementary
*          reflectors
*          = 'F': H = H(1) H(2) . . . H(k) (Forward)
*          = 'B': H = H(k) . . . H(2) H(1) (Backward)
*
*  STOREV  (input) CHARACTER*1
*          Indicates how the vectors which define the elementary
*          reflectors are stored:
*          = 'C': Columnwise
*          = 'R': Rowwise
*
*  M       (input) INTEGER
*          The number of rows of the matrix C.
*
*  N       (input) INTEGER
*          The number of columns of the matrix C.
*
*  K       (input) INTEGER
*          The order of the matrix T (= the number of elementary
*          reflectors whose product defines the block reflector).
*
*  V       (input) DOUBLE PRECISION array, dimension
*                                (LDV,K) if STOREV = 'C'
*                                (LDV,M) if STOREV = 'R' and SIDE = 'L'
*                                (LDV,N) if STOREV = 'R' and SIDE = 'R'
*          The matrix V. See further details.
*
*  LDV     (input) INTEGER
*          The leading dimension of the array V.
*          If STOREV = 'C' and SIDE = 'L', LDV >= max(1,M);
*          if STOREV = 'C' and SIDE = 'R', LDV >= max(1,N);
*          if STOREV = 'R', LDV >= K.
*
*  T       (input) DOUBLE PRECISION array, dimension (LDT,K)
*          The triangular k by k matrix T in the representation of the
*          block reflector.
*
*  LDT     (input) INTEGER
*          The leading dimension of the array T. LDT >= K.
*
*  C       (input/output) DOUBLE PRECISION array, dimension (LDC,N)
*          On entry, the m by n matrix C.
*          On exit, C is overwritten by H*C or H'*C or C*H or C*H'.
*
*  LDC     (input) INTEGER
*          The leading dimension of the array C. LDA >= max(1,M).
*
*  WORK    (workspace) DOUBLE PRECISION array, dimension (LDWORK,K)
*
*  LDWORK  (input) INTEGER
*          The leading dimension of the array WORK.
*          If SIDE = 'L', LDWORK >= max(1,N);
*          if SIDE = 'R', LDWORK >= max(1,M).
*
*  =====================================================================
*
*     .. Parameters ..
      DOUBLE PRECISION   ONE
      PARAMETER          ( ONE = 1.0D+0 )
*     ..
*     .. Local Scalars ..
      CHARACTER          TRANST
      INTEGER            I, J
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      EXTERNAL           LSAME
*     ..
*     .. External Subroutines ..
      EXTERNAL           DCOPY, DGEMM, DTRMM
*     ..
*     .. Executable Statements ..
*
*     Quick return if possible
*
      IF( M.LE.0 .OR. N.LE.0 )
     $   RETURN
*
      IF( LSAME( TRANS, 'N' ) ) THEN
         TRANST = 'T'
      ELSE
         TRANST = 'N'
      END IF
*
      IF( LSAME( STOREV, 'C' ) ) THEN
*
         IF( LSAME( DIRECT, 'F' ) ) THEN
*
*           Let  V =  ( V1 )    (first K rows)
*                     ( V2 )
*           where  V1  is unit lower triangular.
*
            IF( LSAME( SIDE, 'L' ) ) THEN
*
*              Form  H * C  or  H' * C  where  C = ( C1 )
*                                                  ( C2 )
*
*              W := C' * V  =  (C1'*V1 + C2'*V2)  (stored in WORK)
*
*              W := C1'
*
               DO 10 J = 1, K
                  CALL DCOPY( N, C( J, 1 ), LDC, WORK( 1, J ), 1 )
   10          CONTINUE
*
*              W := W * V1
*
               CALL DTRMM( 'Right', 'Lower', 'No transpose', 'Unit', N,
     $                     K, ONE, V, LDV, WORK, LDWORK )
               IF( M.GT.K ) THEN
*
*                 W := W + C2'*V2
*
                  CALL DGEMM( 'Transpose', 'No transpose', N, K, M-K,
     $                        ONE, C( K+1, 1 ), LDC, V( K+1, 1 ), LDV,
     $                        ONE, WORK, LDWORK )
               END IF
*
*              W := W * T'  or  W * T
*
               CALL DTRMM( 'Right', 'Upper', TRANST, 'Non-unit', N, K,
     $                     ONE, T, LDT, WORK, LDWORK )
*
*              C := C - V * W'
*
               IF( M.GT.K ) THEN
*
*                 C2 := C2 - V2 * W'
*
                  CALL DGEMM( 'No transpose', 'Transpose', M-K, N, K,
     $                        -ONE, V( K+1, 1 ), LDV, WORK, LDWORK, ONE,
     $                        C( K+1, 1 ), LDC )
               END IF
*
*              W := W * V1'
*
               CALL DTRMM( 'Right', 'Lower', 'Transpose', 'Unit', N, K,
     $                     ONE, V, LDV, WORK, LDWORK )
*
*              C1 := C1 - W'
*
               DO 30 J = 1, K
                  DO 20 I = 1, N
                     C( J, I ) = C( J, I ) - WORK( I, J )
   20             CONTINUE
   30          CONTINUE
*
            ELSE IF( LSAME( SIDE, 'R' ) ) THEN
*
*              Form  C * H  or  C * H'  where  C = ( C1  C2 )
*
*              W := C * V  =  (C1*V1 + C2*V2)  (stored in WORK)
*
*              W := C1
*
               DO 40 J = 1, K
                  CALL DCOPY( M, C( 1, J ), 1, WORK( 1, J ), 1 )
   40          CONTINUE
*
*              W := W * V1
*
               CALL DTRMM( 'Right', 'Lower', 'No transpose', 'Unit', M,
     $                     K, ONE, V, LDV, WORK, LDWORK )
               IF( N.GT.K ) THEN
*
*                 W := W + C2 * V2
*
                  CALL DGEMM( 'No transpose', 'No transpose', M, K, N-K,
     $                        ONE, C( 1, K+1 ), LDC, V( K+1, 1 ), LDV,
     $                        ONE, WORK, LDWORK )
               END IF
*
*              W := W * T  or  W * T'
*
               CALL DTRMM( 'Right', 'Upper', TRANS, 'Non-unit', M, K,
     $                     ONE, T, LDT, WORK, LDWORK )
*
*              C := C - W * V'
*
               IF( N.GT.K ) THEN
*
*                 C2 := C2 - W * V2'
*
                  CALL DGEMM( 'No transpose', 'Transpose', M, N-K, K,
     $                        -ONE, WORK, LDWORK, V( K+1, 1 ), LDV, ONE,
     $                        C( 1, K+1 ), LDC )
               END IF
*
*              W := W * V1'
*
               CALL DTRMM( 'Right', 'Lower', 'Transpose', 'Unit', M, K,
     $                     ONE, V, LDV, WORK, LDWORK )
*
*              C1 := C1 - W
*
               DO 60 J = 1, K
                  DO 50 I = 1, M
                     C( I, J ) = C( I, J ) - WORK( I, J )
   50             CONTINUE
   60          CONTINUE
            END IF
*
         ELSE
*
*           Let  V =  ( V1 )
*                     ( V2 )    (last K rows)
*           where  V2  is unit upper triangular.
*
            IF( LSAME( SIDE, 'L' ) ) THEN
*
*              Form  H * C  or  H' * C  where  C = ( C1 )
*                                                  ( C2 )
*
*              W := C' * V  =  (C1'*V1 + C2'*V2)  (stored in WORK)
*
*              W := C2'
*
               DO 70 J = 1, K
                  CALL DCOPY( N, C( M-K+J, 1 ), LDC, WORK( 1, J ), 1 )
   70          CONTINUE
*
*              W := W * V2
*
               CALL DTRMM( 'Right', 'Upper', 'No transpose', 'Unit', N,
     $                     K, ONE, V( M-K+1, 1 ), LDV, WORK, LDWORK )
               IF( M.GT.K ) THEN
*
*                 W := W + C1'*V1
*
                  CALL DGEMM( 'Transpose', 'No transpose', N, K, M-K,
     $                        ONE, C, LDC, V, LDV, ONE, WORK, LDWORK )
               END IF
*
*              W := W * T'  or  W * T
*
               CALL DTRMM( 'Right', 'Lower', TRANST, 'Non-unit', N, K,
     $                     ONE, T, LDT, WORK, LDWORK )
*
*              C := C - V * W'
*
               IF( M.GT.K ) THEN
*
*                 C1 := C1 - V1 * W'
*
                  CALL DGEMM( 'No transpose', 'Transpose', M-K, N, K,
     $                        -ONE, V, LDV, WORK, LDWORK, ONE, C, LDC )
               END IF
*
*              W := W * V2'
*
               CALL DTRMM( 'Right', 'Upper', 'Transpose', 'Unit', N, K,
     $                     ONE, V( M-K+1, 1 ), LDV, WORK, LDWORK )
*
*              C2 := C2 - W'
*
               DO 90 J = 1, K
                  DO 80 I = 1, N
                     C( M-K+J, I ) = C( M-K+J, I ) - WORK( I, J )
   80             CONTINUE
   90          CONTINUE