

SGELS(l) ) SGELS(l) NAME
SGELS  solve overdetermined or underdetermined real linear systems involving an MbyN matrix A, or its transpose, using a QR or LQ fac torization of A SYNOPSIS
SUBROUTINE SGELS( TRANS, M, N, NRHS, A, LDA, B, LDB, WORK, LWORK, INFO ) CHARACTER TRANS INTEGER INFO, LDA, LDB, LWORK, M, N, NRHS REAL A( LDA, * ), B( LDB, * ), WORK( * ) PURPOSE
SGELS solves overdetermined or underdetermined real linear systems involving an MbyN matrix A, or its transpose, using a QR or LQ factor ization of A. It is assumed that A has full rank. The following options are provided: 1. If TRANS = 'N' and m >= n: find the least squares solution of an overdetermined system, i.e., solve the least squares problem minimize  B  A*X . 2. If TRANS = 'N' and m < n: find the minimum norm solution of an underdetermined system A * X = B. 3. If TRANS = 'T' and m >= n: find the minimum norm solution of an undetermined system A**T * X = B. 4. If TRANS = 'T' and m < n: find the least squares solution of an overdetermined system, i.e., solve the least squares problem minimize  B  A**T * X . Several right hand side vectors b and solution vectors x can be handled in a single call; they are stored as the columns of the MbyNRHS right hand side matrix B and the NbyNRHS solution matrix X. ARGUMENTS
TRANS (input) CHARACTER = 'N': the linear system involves A; = 'T': the linear system involves A**T. 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. NRHS (input) INTEGER The number of right hand sides, i.e., the number of columns of the matrices B and X. NRHS >=0. A (input/output) REAL array, dimension (LDA,N) On entry, the MbyN matrix A. On exit, if M >= N, A is overwritten by details of its QR factorization as returned by SGEQRF; if M < N, A is overwritten by details of its LQ factorization as returned by SGELQF. LDA (input) INTEGER The leading dimension of the array A. LDA >= max(1,M). B (input/output) REAL array, dimension (LDB,NRHS) On entry, the matrix B of right hand side vectors, stored columnwise; B is MbyNRHS if TRANS = 'N', or NbyNRHS if TRANS = 'T'. On exit, B is overwritten by the solution vectors, stored columnwise: if TRANS = 'N' and m >= n, rows 1 to n of B contain the least squares solution vectors; the residual sum of squares for the solution in each column is given by the sum of squares of elements N+1 to M in that column; if TRANS = 'N' and m < n, rows 1 to N of B contain the minimum norm solution vectors; if TRANS = 'T' and m >= n, rows 1 to M of B contain the minimum norm solution vectors; if TRANS = 'T' and m < n, rows 1 to M of B contain the least squares solution vectors; the residual sum of squares for the solution in each column is given by the sum of squares of elements M+1 to N in that column. LDB (input) INTEGER The leading dimension of the array B. LDB >= MAX(1,M,N). WORK (workspace/output) REAL array, dimension (LWORK) On exit, if INFO = 0, WORK(1) returns the optimal LWORK. LWORK (input) INTEGER The dimension of the array WORK. LWORK >= max( 1, MN + max( MN, NRHS ) ). For optimal performance, LWORK >= max( 1, MN + max( MN, NRHS )*NB ). where MN = min(M,N) and NB is the optimum 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 ith argument had an illegal value LAPACK version 3.0 15 June 2000 SGELS(l)