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ZGEQLF - compute a QL factorization of a complex M-by-N matrix A
SUBROUTINE ZGEQLF( M, N, A, LDA, TAU, WORK, LWORK, INFO ) INTEGER INFO, LDA, LWORK, M, N COMPLEX*16 A( LDA, * ), TAU( * ), WORK( * )
ZGEQLF computes a QL factorization of a complex M-by-N matrix A: A = Q * L.
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, if m >= n, the lower triangle of the subarray A(m-n+1:m,1:n) contains the N-by-N lower triangular matrix L; if m <= n, the elements on and below the (n-m)-th superdiagonal contain the M-by-N lower trapezoidal matrix L; the remaining elements, with the array TAU, represent the unitary matrix Q as a product of elementary reflectors (see Further Details). LDA (input) INTEGER The leading dimen‐ sion 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/output) COMPLEX*16 array, dimension (LWORK) WORK(1) returns the optimal LWORK. LWORK (input) INTEGER The dimension of the array WORK. LWORK >= max(1,N). For opti‐ mum performance LWORK >= N*NB, where NB is the optimal block‐ size. If LWORK = -1, then a workspace query is assumed; the routine only calculates the optimal size of the WORK array, returns this value as the first entry of the WORK array, and no error message related to LWORK is issued by XERBLA. INFO (output) INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value The matrix Q is represented as a product of elementary reflectors 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(m- k+i+1:m) = 0 and v(m-k+i) = 1; v(1:m-k+i-1) is stored on exit in A(1:m- k+i-1,n-k+i), and tau in TAU(i).