SUBROUTINE SLARFT( DIRECT, STOREV, N, K, V, LDV, TAU, T, LDT ) IMPLICIT NONE * * -- LAPACK auxiliary routine (version 3.2) -- * -- LAPACK is a software package provided by Univ. of Tennessee, -- * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..-- * November 2006 * * .. Scalar Arguments .. CHARACTER DIRECT, STOREV INTEGER K, LDT, LDV, N * .. * .. Array Arguments .. REAL T( LDT, * ), TAU( * ), V( LDV, * ) * .. * * Purpose * ======= * * SLARFT forms the triangular factor T of a real block reflector H * of order n, which is defined as a product of k elementary reflectors. * * If DIRECT = 'F', H = H(1) H(2) . . . H(k) and T is upper triangular; * * If DIRECT = 'B', H = H(k) . . . H(2) H(1) and T is lower triangular. * * If STOREV = 'C', the vector which defines the elementary reflector * H(i) is stored in the i-th column of the array V, and * * H = I - V * T * V' * * If STOREV = 'R', the vector which defines the elementary reflector * H(i) is stored in the i-th row of the array V, and * * H = I - V' * T * V * * Arguments * ========= * * DIRECT (input) CHARACTER*1 * Specifies the order in which the elementary reflectors are * multiplied to form the block reflector: * = 'F': H = H(1) H(2) . . . H(k) (Forward) * = 'B': H = H(k) . . . H(2) H(1) (Backward) * * STOREV (input) CHARACTER*1 * Specifies how the vectors which define the elementary * reflectors are stored (see also Further Details): * = 'C': columnwise * = 'R': rowwise * * N (input) INTEGER * The order of the block reflector H. N >= 0. * * K (input) INTEGER * The order of the triangular factor T (= the number of * elementary reflectors). K >= 1. * * V (input/output) REAL array, dimension * (LDV,K) if STOREV = 'C' * (LDV,N) if STOREV = 'R' * The matrix V. See further details. * * LDV (input) INTEGER * The leading dimension of the array V. * If STOREV = 'C', LDV >= max(1,N); if STOREV = 'R', LDV >= K. * * TAU (input) REAL array, dimension (K) * TAU(i) must contain the scalar factor of the elementary * reflector H(i). * * T (output) REAL array, dimension (LDT,K) * The k by k triangular factor T of the block reflector. * If DIRECT = 'F', T is upper triangular; if DIRECT = 'B', T is * lower triangular. The rest of the array is not used. * * LDT (input) INTEGER * The leading dimension of the array T. LDT >= K. * * Further Details * =============== * * The shape of the matrix V and the storage of the vectors which define * the H(i) is best illustrated by the following example with n = 5 and * k = 3. The elements equal to 1 are not stored; the corresponding * array elements are modified but restored on exit. The rest of the * array is not used. * * DIRECT = 'F' and STOREV = 'C': DIRECT = 'F' and STOREV = 'R': * * V = ( 1 ) V = ( 1 v1 v1 v1 v1 ) * ( v1 1 ) ( 1 v2 v2 v2 ) * ( v1 v2 1 ) ( 1 v3 v3 ) * ( v1 v2 v3 ) * ( v1 v2 v3 ) * * DIRECT = 'B' and STOREV = 'C': DIRECT = 'B' and STOREV = 'R': * * V = ( v1 v2 v3 ) V = ( v1 v1 1 ) * ( v1 v2 v3 ) ( v2 v2 v2 1 ) * ( 1 v2 v3 ) ( v3 v3 v3 v3 1 ) * ( 1 v3 ) * ( 1 ) * * ===================================================================== * * .. Parameters .. REAL ONE, ZERO PARAMETER ( ONE = 1.0E+0, ZERO = 0.0E+0 ) * .. * .. Local Scalars .. INTEGER I, J, PREVLASTV, LASTV REAL VII * .. * .. External Subroutines .. EXTERNAL SGEMV, STRMV * .. * .. External Functions .. LOGICAL LSAME EXTERNAL LSAME * .. * .. Executable Statements .. * * Quick return if possible * IF( N.EQ.0 ) \$ RETURN * IF( LSAME( DIRECT, 'F' ) ) THEN PREVLASTV = N DO 20 I = 1, K PREVLASTV = MAX( I, PREVLASTV ) IF( TAU( I ).EQ.ZERO ) THEN * * H(i) = I * DO 10 J = 1, I T( J, I ) = ZERO 10 CONTINUE ELSE * * general case * VII = V( I, I ) V( I, I ) = ONE IF( LSAME( STOREV, 'C' ) ) THEN ! Skip any trailing zeros. DO LASTV = N, I+1, -1 IF( V( LASTV, I ).NE.ZERO ) EXIT END DO J = MIN( LASTV, PREVLASTV ) * * T(1:i-1,i) := - tau(i) * V(i:j,1:i-1)' * V(i:j,i) * CALL SGEMV( 'Transpose', J-I+1, I-1, -TAU( I ), \$ V( I, 1 ), LDV, V( I, I ), 1, ZERO, \$ T( 1, I ), 1 ) ELSE ! Skip any trailing zeros. DO LASTV = N, I+1, -1 IF( V( I, LASTV ).NE.ZERO ) EXIT END DO J = MIN( LASTV, PREVLASTV ) * * T(1:i-1,i) := - tau(i) * V(1:i-1,i:j) * V(i,i:j)' * CALL SGEMV( 'No transpose', I-1, J-I+1, -TAU( I ), \$ V( 1, I ), LDV, V( I, I ), LDV, ZERO, \$ T( 1, I ), 1 ) END IF V( I, I ) = VII * * T(1:i-1,i) := T(1:i-1,1:i-1) * T(1:i-1,i) * CALL STRMV( 'Upper', 'No transpose', 'Non-unit', I-1, T, \$ LDT, T( 1, I ), 1 ) T( I, I ) = TAU( I ) IF( I.GT.1 ) THEN PREVLASTV = MAX( PREVLASTV, LASTV ) ELSE PREVLASTV = LASTV END IF END IF 20 CONTINUE ELSE PREVLASTV = 1 DO 40 I = K, 1, -1 IF( TAU( I ).EQ.ZERO ) THEN * * H(i) = I * DO 30 J = I, K T( J, I ) = ZERO 30 CONTINUE ELSE * * general case * IF( I.LT.K ) THEN IF( LSAME( STOREV, 'C' ) ) THEN VII = V( N-K+I, I ) V( N-K+I, I ) = ONE ! Skip any leading zeros. DO LASTV = 1, I-1 IF( V( LASTV, I ).NE.ZERO ) EXIT END DO J = MAX( LASTV, PREVLASTV ) * * T(i+1:k,i) := * - tau(i) * V(j:n-k+i,i+1:k)' * V(j:n-k+i,i) * CALL SGEMV( 'Transpose', N-K+I-J+1, K-I, -TAU( I ), \$ V( J, I+1 ), LDV, V( J, I ), 1, ZERO, \$ T( I+1, I ), 1 ) V( N-K+I, I ) = VII ELSE VII = V( I, N-K+I ) V( I, N-K+I ) = ONE ! Skip any leading zeros. DO LASTV = 1, I-1 IF( V( I, LASTV ).NE.ZERO ) EXIT END DO J = MAX( LASTV, PREVLASTV ) * * T(i+1:k,i) := * - tau(i) * V(i+1:k,j:n-k+i) * V(i,j:n-k+i)' * CALL SGEMV( 'No transpose', K-I, N-K+I-J+1, \$ -TAU( I ), V( I+1, J ), LDV, V( I, J ), LDV, \$ ZERO, T( I+1, I ), 1 ) V( I, N-K+I ) = VII END IF * * T(i+1:k,i) := T(i+1:k,i+1:k) * T(i+1:k,i) * CALL STRMV( 'Lower', 'No transpose', 'Non-unit', K-I, \$ T( I+1, I+1 ), LDT, T( I+1, I ), 1 ) IF( I.GT.1 ) THEN PREVLASTV = MIN( PREVLASTV, LASTV ) ELSE PREVLASTV = LASTV END IF END IF T( I, I ) = TAU( I ) END IF 40 CONTINUE END IF RETURN * * End of SLARFT * END