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### NAME

ZGBSVX - use the LU factorization to compute the solution to a complex system of linear equations A * X = B, A**T * X = B, or A**H * X = B,

### SYNOPSIS

SUBROUTINE ZGBSVX( FACT, TRANS, N, KL, KU, NRHS, AB, LDAB, AFB, LDAFB, IPIV, EQUED, R, C, B, LDB, X, LDX, RCOND, FERR, BERR, WORK, RWORK, INFO ) CHARACTER EQUED, FACT, TRANS INTEGER INFO, KL, KU, LDAB, LDAFB, LDB, LDX, N, NRHS DOUBLE PRECISION RCOND INTEGER IPIV( * ) DOUBLE PRECISION BERR( * ), C( * ), FERR( * ), R( * ), RWORK( * ) COMPLEX*16 AB( LDAB, * ), AFB( LDAFB, * ), B( LDB, * ), WORK( * ), X( LDX, * )

### PURPOSE

ZGBSVX uses the LU factorization to compute the solution to a complex system of linear equations A * X = B, A**T * X = B, or A**H * X = B, where A is a band matrix of order N with KL subdiagonals and KU super‐ diagonals, and X and B are N-by-NRHS matrices. Error bounds on the solution and a condition estimate are also pro‐ vided.

### DESCRIPTION

The following steps are performed by this subroutine: 1. If FACT = ’E’, real scaling factors are computed to equilibrate the system: TRANS = ’N’: diag(R)*A*diag(C) *inv(diag(C))*X = diag(R)*B TRANS = ’T’: (diag(R)*A*diag(C))**T *inv(diag(R))*X = diag(C)*B TRANS = ’C’: (diag(R)*A*diag(C))**H *inv(diag(R))*X = diag(C)*B Whether or not the system will be equilibrated depends on the scaling of the matrix A, but if equilibration is used, A is overwritten by diag(R)*A*diag(C) and B by diag(R)*B (if TRANS=’N’) or diag(C)*B (if TRANS = ’T’ or ’C’). 2. If FACT = ’N’ or ’E’, the LU decomposition is used to factor the matrix A (after equilibration if FACT = ’E’) as A = L * U, where L is a product of permutation and unit lower triangular matrices with KL subdiagonals, and U is upper triangular with KL+KU superdiagonals. 3. If some U(i,i)=0, so that U is exactly singular, then the routine returns with INFO = i. Otherwise, the factored form of A is used to estimate the condition number of the matrix A. If the reciprocal of the condition number is less than machine precision, INFO = N+1 is returned as a warning, but the routine still goes on to solve for X and compute error bounds as described below. 4. The system of equations is solved for X using the factored form of A. 5. Iterative refinement is applied to improve the computed solution matrix and calculate error bounds and backward error estimates for it. 6. If equilibration was used, the matrix X is premultiplied by diag(C) (if TRANS = ’N’) or diag(R) (if TRANS = ’T’ or ’C’) so that it solves the original system before equilibration.

### ARGUMENTS

FACT (input) CHARACTER*1 Specifies whether or not the factored form of the matrix A is supplied on entry, and if not, whether the matrix A should be equilibrated before it is factored. = ’F’: On entry, AFB and IPIV contain the factored form of A. If EQUED is not ’N’, the matrix A has been equilibrated with scaling factors given by R and C. AB, AFB, and IPIV are not modified. = ’N’: The matrix A will be copied to AFB and factored. = ’E’: The matrix A will be equilibrated if necessary, then copied to AFB and factored. TRANS (input) CHARACTER*1 Specifies the form of the system of equations. = ’N’: A * X = B (No transpose) = ’T’: A**T * X = B (Transpose) = ’C’: A**H * X = B (Conjugate transpose) N (input) INTEGER The number of linear equations, i.e., the order of the matrix A. N >= 0. KL (input) INTEGER The number of subdiagonals within the band of A. KL >= 0. KU (input) INTEGER The number of superdiagonals within the band of A. KU >= 0. NRHS (input) INTEGER The number of right hand sides, i.e., the number of columns of the matrices B and X. NRHS >= 0. AB (input/output) COMPLEX*16 array, dimension (LDAB,N) On entry, the matrix A in band storage, in rows 1 to KL+KU+1. The j-th column of A is stored in the j-th column of the array AB as follows: AB(KU+1+i-j,j) = A(i,j) for max(1,j- KU)<=i<=min(N,j+kl) If FACT = ’F’ and EQUED is not ’N’, then A must have been equi‐ librated by the scaling factors in R and/or C. AB is not modi‐ fied if FACT = ’F’ or ’N’, or if FACT = ’E’ and EQUED = ’N’ on exit. On exit, if EQUED .ne. ’N’, A is scaled as follows: EQUED = ’R’: A := diag(R) * A EQUED = ’C’: A := A * diag(C) EQUED = ’B’: A := diag(R) * A * diag(C). LDAB (input) INTEGER The leading dimension of the array AB. LDAB >= KL+KU+1. AFB (input or output) COMPLEX*16 array, dimension (LDAFB,N) If FACT = ’F’, then AFB is an input argument and on entry con‐ tains details of the LU factorization of the band matrix A, as computed by ZGBTRF. U is stored as an upper triangular band matrix with KL+KU superdiagonals in rows 1 to KL+KU+1, and the multipliers used during the factorization are stored in rows KL+KU+2 to 2*KL+KU+1. If EQUED .ne. ’N’, then AFB is the fac‐ tored form of the equilibrated matrix A. If FACT = ’N’, then AFB is an output argument and on exit returns details of the LU factorization of A. If FACT = ’E’, then AFB is an output argument and on exit returns details of the LU factorization of the equilibrated matrix A (see the description of AB for the form of the equili‐ brated matrix). LDAFB (input) INTEGER The leading dimension of the array AFB. LDAFB >= 2*KL+KU+1. IPIV (input or output) INTEGER array, dimension (N) If FACT = ’F’, then IPIV is an input argument and on entry con‐ tains the pivot indices from the factorization A = L*U as com‐ puted by ZGBTRF; row i of the matrix was interchanged with row IPIV(i). If FACT = ’N’, then IPIV is an output argument and on exit con‐ tains the pivot indices from the factorization A = L*U of the original matrix A. If FACT = ’E’, then IPIV is an output argument and on exit con‐ tains the pivot indices from the factorization A = L*U of the equilibrated matrix A. EQUED (input or output) CHARACTER*1 Specifies the form of equilibration that was done. = ’N’: No equilibration (always true if FACT = ’N’). = ’R’: Row equilibration, i.e., A has been premultiplied by diag(R). = ’C’: Column equilibration, i.e., A has been post‐ multiplied by diag(C). = ’B’: Both row and column equilibra‐ tion, i.e., A has been replaced by diag(R) * A * diag(C). EQUED is an input argument if FACT = ’F’; otherwise, it is an output argument. R (input or output) DOUBLE PRECISION array, dimension (N) The row scale factors for A. If EQUED = ’R’ or ’B’, A is mul‐ tiplied on the left by diag(R); if EQUED = ’N’ or ’C’, R is not accessed. R is an input argument if FACT = ’F’; otherwise, R is an output argument. If FACT = ’F’ and EQUED = ’R’ or ’B’, each element of R must be positive. C (input or output) DOUBLE PRECISION array, dimension (N) The column scale factors for A. If EQUED = ’C’ or ’B’, A is multiplied on the right by diag(C); if EQUED = ’N’ or ’R’, C is not accessed. C is an input argument if FACT = ’F’; otherwise, C is an output argument. If FACT = ’F’ and EQUED = ’C’ or ’B’, each element of C must be positive. B (input/output) COMPLEX*16 array, dimension (LDB,NRHS) On entry, the right hand side matrix B. On exit, if EQUED = ’N’, B is not modified; if TRANS = ’N’ and EQUED = ’R’ or ’B’, B is overwritten by diag(R)*B; if TRANS = ’T’ or ’C’ and EQUED = ’C’ or ’B’, B is overwritten by diag(C)*B. LDB (input) INTEGER The leading dimension of the array B. LDB >= max(1,N). X (output) COMPLEX*16 array, dimension (LDX,NRHS) If INFO = 0 or INFO = N+1, the N-by-NRHS solution matrix X to the original system of equations. Note that A and B are modi‐ fied on exit if EQUED .ne. ’N’, and the solution to the equilibrated system is inv(diag(C))*X if TRANS = ’N’ and EQUED = ’C’ or ’B’, or inv(diag(R))*X if TRANS = ’T’ or ’C’ and EQUED = ’R’ or ’B’. LDX (input) INTEGER The leading dimension of the array X. LDX >= max(1,N). RCOND (output) DOUBLE PRECISION The estimate of the reciprocal condition number of the matrix A after equilibration (if done). If RCOND is less than the machine precision (in particular, if RCOND = 0), the matrix is singular to working precision. This condition is indicated by a return code of INFO > 0. FERR (output) DOUBLE PRECISION array, dimension (NRHS) The estimated forward error bound for each solution vector X(j) (the j-th column of the solution matrix X). If XTRUE is the true solution corresponding to X(j), FERR(j) is an estimated upper bound for the magnitude of the largest element in (X(j) - XTRUE) divided by the magnitude of the largest element in X(j). The estimate is as reliable as the estimate for RCOND, and is almost always a slight overestimate of the true error. BERR (output) DOUBLE PRECISION array, dimension (NRHS) The componentwise relative backward error of each solution vec‐ tor X(j) (i.e., the smallest relative change in any element of A or B that makes X(j) an exact solution). WORK (workspace) COMPLEX*16 array, dimension (2*N) RWORK (workspace/output) DOUBLE PRECISION array, dimension (N) RWORK(1) contains the reciprocal pivot growth factor norm(A)/norm(U). The "max absolute element" norm is used. If RWORK(1) is much less than 1, then the stability of the LU fac‐ torization of the (equilibrated) matrix A could be poor. This also means that the solution X, condition estimator RCOND, and forward error bound FERR could be unreliable. If factorization RWORK(1) contains the reciprocal pivot growth factor for the leading INFO columns of A. INFO (output) INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value > 0: if INFO = i, and i is <= N: U(i,i) is exactly zero. The factorization has been com‐ pleted, but the factor U is exactly singular, so the solution and error bounds could not be computed. RCOND = 0 is returned. = N+1: U is nonsingular, but RCOND is less than machine preci‐ sion, meaning that the matrix is singular to working precision. Nevertheless, the solution and error bounds are computed because there are a number of situations where the computed solution can be more accurate than the value of RCOND would suggest.