SUBROUTINE ZGELQ2( M, N, A, LDA, TAU, WORK, INFO ) * * -- LAPACK routine (version 3.2.2) -- * -- LAPACK is a software package provided by Univ. of Tennessee, -- * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..-- * June 2010 * * .. Scalar Arguments .. INTEGER INFO, LDA, M, N * .. * .. Array Arguments .. COMPLEX*16 A( LDA, * ), TAU( * ), WORK( * ) * .. * * Purpose * ======= * * ZGELQ2 computes an LQ factorization of a complex m by n matrix A: * A = L * Q. * * Arguments * ========= * * M (input) INTEGER * The number of rows of the matrix A. M >= 0. * * N (input) INTEGER * The number of columns of the matrix A. N >= 0. * * A (input/output) COMPLEX*16 array, dimension (LDA,N) * On entry, the m by n matrix A. * On exit, the elements on and below the diagonal of the array * contain the m by min(m,n) lower trapezoidal matrix L (L is * lower triangular if m <= n); the elements above the diagonal, * with the array TAU, represent the unitary matrix Q as a * product of elementary reflectors (see Further Details). * * LDA (input) INTEGER * The leading dimension of the array A. LDA >= max(1,M). * * TAU (output) COMPLEX*16 array, dimension (min(M,N)) * The scalar factors of the elementary reflectors (see Further * Details). * * WORK (workspace) COMPLEX*16 array, dimension (M) * * INFO (output) INTEGER * = 0: successful exit * < 0: if INFO = -i, the i-th argument had an illegal value * * Further Details * =============== * * The matrix Q is represented as a product of elementary reflectors * * Q = H(k)' . . . H(2)' H(1)', where k = min(m,n). * * Each H(i) has the form * * H(i) = I - tau * v * v' * * where tau is a complex scalar, and v is a complex vector with * v(1:i-1) = 0 and v(i) = 1; conjg(v(i+1:n)) is stored on exit in * A(i,i+1:n), and tau in TAU(i). * * ===================================================================== * * .. Parameters .. COMPLEX*16 ONE PARAMETER ( ONE = ( 1.0D+0, 0.0D+0 ) ) * .. * .. Local Scalars .. INTEGER I, K COMPLEX*16 ALPHA * .. * .. External Subroutines .. EXTERNAL XERBLA, ZLACGV, ZLARF, ZLARFG * .. * .. Intrinsic Functions .. INTRINSIC MAX, MIN * .. * .. Executable Statements .. * * Test the input arguments * INFO = 0 IF( M.LT.0 ) THEN INFO = -1 ELSE IF( N.LT.0 ) THEN INFO = -2 ELSE IF( LDA.LT.MAX( 1, M ) ) THEN INFO = -4 END IF IF( INFO.NE.0 ) THEN CALL XERBLA( 'ZGELQ2', -INFO ) RETURN END IF * K = MIN( M, N ) * DO 10 I = 1, K * * Generate elementary reflector H(i) to annihilate A(i,i+1:n) * CALL ZLACGV( N-I+1, A( I, I ), LDA ) ALPHA = A( I, I ) CALL ZLARFG( N-I+1, ALPHA, A( I, MIN( I+1, N ) ), LDA, \$ TAU( I ) ) IF( I.LT.M ) THEN * * Apply H(i) to A(i+1:m,i:n) from the right * A( I, I ) = ONE CALL ZLARF( 'Right', M-I, N-I+1, A( I, I ), LDA, TAU( I ), \$ A( I+1, I ), LDA, WORK ) END IF A( I, I ) = ALPHA CALL ZLACGV( N-I+1, A( I, I ), LDA ) 10 CONTINUE RETURN * * End of ZGELQ2 * END