642 lines
		
	
	
		
			22 KiB
		
	
	
	
		
			Fortran
		
	
	
	
			
		
		
	
	
			642 lines
		
	
	
		
			22 KiB
		
	
	
	
		
			Fortran
		
	
	
	
| *> \brief <b> SGBSVX computes the solution to system of linear equations A * X = B for GB matrices</b>
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| *
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| *  =========== DOCUMENTATION ===========
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| *
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| * Online html documentation available at
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| *            http://www.netlib.org/lapack/explore-html/
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| *
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| *> \htmlonly
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| *> Download SGBSVX + dependencies
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| *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.tgz?format=tgz&filename=/lapack/lapack_routine/sgbsvx.f">
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| *> [TGZ]</a>
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| *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/sgbsvx.f">
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| *> [ZIP]</a>
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| *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/sgbsvx.f">
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| *> [TXT]</a>
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| *> \endhtmlonly
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| *
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| *  Definition:
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| *  ===========
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| *
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| *       SUBROUTINE SGBSVX( FACT, TRANS, N, KL, KU, NRHS, AB, LDAB, AFB,
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| *                          LDAFB, IPIV, EQUED, R, C, B, LDB, X, LDX,
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| *                          RCOND, FERR, BERR, WORK, IWORK, INFO )
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| *
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| *       .. Scalar Arguments ..
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| *       CHARACTER          EQUED, FACT, TRANS
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| *       INTEGER            INFO, KL, KU, LDAB, LDAFB, LDB, LDX, N, NRHS
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| *       REAL               RCOND
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| *       ..
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| *       .. Array Arguments ..
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| *       INTEGER            IPIV( * ), IWORK( * )
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| *       REAL               AB( LDAB, * ), AFB( LDAFB, * ), B( LDB, * ),
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| *      $                   BERR( * ), C( * ), FERR( * ), R( * ),
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| *      $                   WORK( * ), X( LDX, * )
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| *       ..
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| *
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| *
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| *> \par Purpose:
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| *  =============
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| *>
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| *> \verbatim
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| *>
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| *> SGBSVX uses the LU factorization to compute the solution to a real
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| *> system of linear equations A * X = B, A**T * X = B, or A**H * X = B,
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| *> where A is a band matrix of order N with KL subdiagonals and KU
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| *> superdiagonals, and X and B are N-by-NRHS matrices.
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| *>
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| *> Error bounds on the solution and a condition estimate are also
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| *> provided.
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| *> \endverbatim
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| *
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| *> \par Description:
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| *  =================
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| *>
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| *> \verbatim
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| *>
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| *> The following steps are performed by this subroutine:
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| *>
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| *> 1. If FACT = 'E', real scaling factors are computed to equilibrate
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| *>    the system:
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| *>       TRANS = 'N':  diag(R)*A*diag(C)     *inv(diag(C))*X = diag(R)*B
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| *>       TRANS = 'T': (diag(R)*A*diag(C))**T *inv(diag(R))*X = diag(C)*B
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| *>       TRANS = 'C': (diag(R)*A*diag(C))**H *inv(diag(R))*X = diag(C)*B
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| *>    Whether or not the system will be equilibrated depends on the
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| *>    scaling of the matrix A, but if equilibration is used, A is
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| *>    overwritten by diag(R)*A*diag(C) and B by diag(R)*B (if TRANS='N')
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| *>    or diag(C)*B (if TRANS = 'T' or 'C').
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| *>
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| *> 2. If FACT = 'N' or 'E', the LU decomposition is used to factor the
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| *>    matrix A (after equilibration if FACT = 'E') as
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| *>       A = L * U,
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| *>    where L is a product of permutation and unit lower triangular
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| *>    matrices with KL subdiagonals, and U is upper triangular with
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| *>    KL+KU superdiagonals.
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| *>
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| *> 3. If some U(i,i)=0, so that U is exactly singular, then the routine
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| *>    returns with INFO = i. Otherwise, the factored form of A is used
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| *>    to estimate the condition number of the matrix A.  If the
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| *>    reciprocal of the condition number is less than machine precision,
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| *>    INFO = N+1 is returned as a warning, but the routine still goes on
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| *>    to solve for X and compute error bounds as described below.
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| *>
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| *> 4. The system of equations is solved for X using the factored form
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| *>    of A.
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| *>
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| *> 5. Iterative refinement is applied to improve the computed solution
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| *>    matrix and calculate error bounds and backward error estimates
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| *>    for it.
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| *>
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| *> 6. If equilibration was used, the matrix X is premultiplied by
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| *>    diag(C) (if TRANS = 'N') or diag(R) (if TRANS = 'T' or 'C') so
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| *>    that it solves the original system before equilibration.
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| *> \endverbatim
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| *
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| *  Arguments:
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| *  ==========
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| *
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| *> \param[in] FACT
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| *> \verbatim
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| *>          FACT is CHARACTER*1
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| *>          Specifies whether or not the factored form of the matrix A is
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| *>          supplied on entry, and if not, whether the matrix A should be
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| *>          equilibrated before it is factored.
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| *>          = 'F':  On entry, AFB and IPIV contain the factored form of
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| *>                  A.  If EQUED is not 'N', the matrix A has been
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| *>                  equilibrated with scaling factors given by R and C.
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| *>                  AB, AFB, and IPIV are not modified.
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| *>          = 'N':  The matrix A will be copied to AFB and factored.
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| *>          = 'E':  The matrix A will be equilibrated if necessary, then
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| *>                  copied to AFB and factored.
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| *> \endverbatim
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| *>
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| *> \param[in] TRANS
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| *> \verbatim
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| *>          TRANS is CHARACTER*1
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| *>          Specifies the form of the system of equations.
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| *>          = 'N':  A * X = B     (No transpose)
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| *>          = 'T':  A**T * X = B  (Transpose)
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| *>          = 'C':  A**H * X = B  (Transpose)
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| *> \endverbatim
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| *>
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| *> \param[in] N
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| *> \verbatim
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| *>          N is INTEGER
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| *>          The number of linear equations, i.e., the order of the
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| *>          matrix A.  N >= 0.
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| *> \endverbatim
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| *>
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| *> \param[in] KL
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| *> \verbatim
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| *>          KL is INTEGER
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| *>          The number of subdiagonals within the band of A.  KL >= 0.
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| *> \endverbatim
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| *>
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| *> \param[in] KU
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| *> \verbatim
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| *>          KU is INTEGER
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| *>          The number of superdiagonals within the band of A.  KU >= 0.
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| *> \endverbatim
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| *>
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| *> \param[in] NRHS
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| *> \verbatim
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| *>          NRHS is INTEGER
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| *>          The number of right hand sides, i.e., the number of columns
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| *>          of the matrices B and X.  NRHS >= 0.
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| *> \endverbatim
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| *>
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| *> \param[in,out] AB
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| *> \verbatim
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| *>          AB is REAL array, dimension (LDAB,N)
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| *>          On entry, the matrix A in band storage, in rows 1 to KL+KU+1.
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| *>          The j-th column of A is stored in the j-th column of the
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| *>          array AB as follows:
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| *>          AB(KU+1+i-j,j) = A(i,j) for max(1,j-KU)<=i<=min(N,j+kl)
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| *>
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| *>          If FACT = 'F' and EQUED is not 'N', then A must have been
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| *>          equilibrated by the scaling factors in R and/or C.  AB is not
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| *>          modified if FACT = 'F' or 'N', or if FACT = 'E' and
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| *>          EQUED = 'N' on exit.
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| *>
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| *>          On exit, if EQUED .ne. 'N', A is scaled as follows:
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| *>          EQUED = 'R':  A := diag(R) * A
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| *>          EQUED = 'C':  A := A * diag(C)
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| *>          EQUED = 'B':  A := diag(R) * A * diag(C).
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| *> \endverbatim
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| *>
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| *> \param[in] LDAB
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| *> \verbatim
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| *>          LDAB is INTEGER
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| *>          The leading dimension of the array AB.  LDAB >= KL+KU+1.
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| *> \endverbatim
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| *>
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| *> \param[in,out] AFB
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| *> \verbatim
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| *>          AFB is REAL array, dimension (LDAFB,N)
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| *>          If FACT = 'F', then AFB is an input argument and on entry
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| *>          contains details of the LU factorization of the band matrix
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| *>          A, as computed by SGBTRF.  U is stored as an upper triangular
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| *>          band matrix with KL+KU superdiagonals in rows 1 to KL+KU+1,
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| *>          and the multipliers used during the factorization are stored
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| *>          in rows KL+KU+2 to 2*KL+KU+1.  If EQUED .ne. 'N', then AFB is
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| *>          the factored form of the equilibrated matrix A.
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| *>
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| *>          If FACT = 'N', then AFB is an output argument and on exit
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| *>          returns details of the LU factorization of A.
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| *>
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| *>          If FACT = 'E', then AFB is an output argument and on exit
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| *>          returns details of the LU factorization of the equilibrated
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| *>          matrix A (see the description of AB for the form of the
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| *>          equilibrated matrix).
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| *> \endverbatim
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| *>
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| *> \param[in] LDAFB
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| *> \verbatim
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| *>          LDAFB is INTEGER
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| *>          The leading dimension of the array AFB.  LDAFB >= 2*KL+KU+1.
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| *> \endverbatim
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| *>
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| *> \param[in,out] IPIV
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| *> \verbatim
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| *>          IPIV is INTEGER array, dimension (N)
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| *>          If FACT = 'F', then IPIV is an input argument and on entry
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| *>          contains the pivot indices from the factorization A = L*U
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| *>          as computed by SGBTRF; row i of the matrix was interchanged
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| *>          with row IPIV(i).
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| *>
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| *>          If FACT = 'N', then IPIV is an output argument and on exit
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| *>          contains the pivot indices from the factorization A = L*U
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| *>          of the original matrix A.
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| *>
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| *>          If FACT = 'E', then IPIV is an output argument and on exit
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| *>          contains the pivot indices from the factorization A = L*U
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| *>          of the equilibrated matrix A.
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| *> \endverbatim
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| *>
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| *> \param[in,out] EQUED
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| *> \verbatim
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| *>          EQUED is CHARACTER*1
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| *>          Specifies the form of equilibration that was done.
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| *>          = 'N':  No equilibration (always true if FACT = 'N').
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| *>          = 'R':  Row equilibration, i.e., A has been premultiplied by
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| *>                  diag(R).
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| *>          = 'C':  Column equilibration, i.e., A has been postmultiplied
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| *>                  by diag(C).
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| *>          = 'B':  Both row and column equilibration, i.e., A has been
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| *>                  replaced by diag(R) * A * diag(C).
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| *>          EQUED is an input argument if FACT = 'F'; otherwise, it is an
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| *>          output argument.
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| *> \endverbatim
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| *>
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| *> \param[in,out] R
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| *> \verbatim
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| *>          R is REAL array, dimension (N)
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| *>          The row scale factors for A.  If EQUED = 'R' or 'B', A is
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| *>          multiplied on the left by diag(R); if EQUED = 'N' or 'C', R
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| *>          is not accessed.  R is an input argument if FACT = 'F';
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| *>          otherwise, R is an output argument.  If FACT = 'F' and
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| *>          EQUED = 'R' or 'B', each element of R must be positive.
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| *> \endverbatim
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| *>
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| *> \param[in,out] C
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| *> \verbatim
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| *>          C is REAL array, dimension (N)
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| *>          The column scale factors for A.  If EQUED = 'C' or 'B', A is
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| *>          multiplied on the right by diag(C); if EQUED = 'N' or 'R', C
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| *>          is not accessed.  C is an input argument if FACT = 'F';
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| *>          otherwise, C is an output argument.  If FACT = 'F' and
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| *>          EQUED = 'C' or 'B', each element of C must be positive.
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| *> \endverbatim
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| *>
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| *> \param[in,out] B
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| *> \verbatim
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| *>          B is REAL array, dimension (LDB,NRHS)
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| *>          On entry, the right hand side matrix B.
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| *>          On exit,
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| *>          if EQUED = 'N', B is not modified;
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| *>          if TRANS = 'N' and EQUED = 'R' or 'B', B is overwritten by
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| *>          diag(R)*B;
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| *>          if TRANS = 'T' or 'C' and EQUED = 'C' or 'B', B is
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| *>          overwritten by diag(C)*B.
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| *> \endverbatim
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| *>
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| *> \param[in] LDB
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| *> \verbatim
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| *>          LDB is INTEGER
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| *>          The leading dimension of the array B.  LDB >= max(1,N).
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| *> \endverbatim
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| *>
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| *> \param[out] X
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| *> \verbatim
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| *>          X is REAL array, dimension (LDX,NRHS)
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| *>          If INFO = 0 or INFO = N+1, the N-by-NRHS solution matrix X
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| *>          to the original system of equations.  Note that A and B are
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| *>          modified on exit if EQUED .ne. 'N', and the solution to the
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| *>          equilibrated system is inv(diag(C))*X if TRANS = 'N' and
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| *>          EQUED = 'C' or 'B', or inv(diag(R))*X if TRANS = 'T' or 'C'
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| *>          and EQUED = 'R' or 'B'.
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| *> \endverbatim
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| *>
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| *> \param[in] LDX
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| *> \verbatim
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| *>          LDX is INTEGER
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| *>          The leading dimension of the array X.  LDX >= max(1,N).
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| *> \endverbatim
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| *>
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| *> \param[out] RCOND
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| *> \verbatim
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| *>          RCOND is REAL
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| *>          The estimate of the reciprocal condition number of the matrix
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| *>          A after equilibration (if done).  If RCOND is less than the
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| *>          machine precision (in particular, if RCOND = 0), the matrix
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| *>          is singular to working precision.  This condition is
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| *>          indicated by a return code of INFO > 0.
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| *> \endverbatim
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| *>
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| *> \param[out] FERR
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| *> \verbatim
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| *>          FERR is REAL array, dimension (NRHS)
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| *>          The estimated forward error bound for each solution vector
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| *>          X(j) (the j-th column of the solution matrix X).
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| *>          If XTRUE is the true solution corresponding to X(j), FERR(j)
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| *>          is an estimated upper bound for the magnitude of the largest
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| *>          element in (X(j) - XTRUE) divided by the magnitude of the
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| *>          largest element in X(j).  The estimate is as reliable as
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| *>          the estimate for RCOND, and is almost always a slight
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| *>          overestimate of the true error.
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| *> \endverbatim
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| *>
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| *> \param[out] BERR
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| *> \verbatim
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| *>          BERR is REAL array, dimension (NRHS)
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| *>          The componentwise relative backward error of each solution
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| *>          vector X(j) (i.e., the smallest relative change in
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| *>          any element of A or B that makes X(j) an exact solution).
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| *> \endverbatim
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| *>
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| *> \param[out] WORK
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| *> \verbatim
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| *>          WORK is REAL array, dimension (3*N)
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| *>          On exit, WORK(1) contains the reciprocal pivot growth
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| *>          factor norm(A)/norm(U). The "max absolute element" norm is
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| *>          used. If WORK(1) is much less than 1, then the stability
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| *>          of the LU factorization of the (equilibrated) matrix A
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| *>          could be poor. This also means that the solution X, condition
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| *>          estimator RCOND, and forward error bound FERR could be
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| *>          unreliable. If factorization fails with 0<INFO<=N, then
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| *>          WORK(1) contains the reciprocal pivot growth factor for the
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| *>          leading INFO columns of A.
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| *> \endverbatim
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| *>
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| *> \param[out] IWORK
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| *> \verbatim
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| *>          IWORK is INTEGER array, dimension (N)
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| *> \endverbatim
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| *>
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| *> \param[out] INFO
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| *> \verbatim
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| *>          INFO is INTEGER
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| *>          = 0:  successful exit
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| *>          < 0:  if INFO = -i, the i-th argument had an illegal value
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| *>          > 0:  if INFO = i, and i is
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| *>                <= N:  U(i,i) is exactly zero.  The factorization
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| *>                       has been completed, but the factor U is exactly
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| *>                       singular, so the solution and error bounds
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| *>                       could not be computed. RCOND = 0 is returned.
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| *>                = N+1: U is nonsingular, but RCOND is less than machine
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| *>                       precision, meaning that the matrix is singular
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| *>                       to working precision.  Nevertheless, the
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| *>                       solution and error bounds are computed because
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| *>                       there are a number of situations where the
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| *>                       computed solution can be more accurate than the
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| *> \endverbatim
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| *
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| *  Authors:
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| *  ========
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| *
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| *> \author Univ. of Tennessee
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| *> \author Univ. of California Berkeley
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| *> \author Univ. of Colorado Denver
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| *> \author NAG Ltd.
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| *
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| *> \ingroup realGBsolve
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| *
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| *  =====================================================================
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|       SUBROUTINE SGBSVX( FACT, TRANS, N, KL, KU, NRHS, AB, LDAB, AFB,
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|      $                   LDAFB, IPIV, EQUED, R, C, B, LDB, X, LDX,
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|      $                   RCOND, FERR, BERR, WORK, IWORK, INFO )
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| *
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| *  -- LAPACK driver routine --
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| *  -- LAPACK is a software package provided by Univ. of Tennessee,    --
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| *  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
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| *
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| *     .. Scalar Arguments ..
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|       CHARACTER          EQUED, FACT, TRANS
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|       INTEGER            INFO, KL, KU, LDAB, LDAFB, LDB, LDX, N, NRHS
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|       REAL               RCOND
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| *     ..
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| *     .. Array Arguments ..
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|       INTEGER            IPIV( * ), IWORK( * )
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|       REAL               AB( LDAB, * ), AFB( LDAFB, * ), B( LDB, * ),
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|      $                   BERR( * ), C( * ), FERR( * ), R( * ),
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|      $                   WORK( * ), X( LDX, * )
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| *     ..
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| *
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| *  =====================================================================
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| *  Moved setting of INFO = N+1 so INFO does not subsequently get
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| *  overwritten.  Sven, 17 Mar 05.
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| *  =====================================================================
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| *
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| *     .. Parameters ..
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|       REAL               ZERO, ONE
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|       PARAMETER          ( ZERO = 0.0E+0, ONE = 1.0E+0 )
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| *     ..
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| *     .. Local Scalars ..
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|       LOGICAL            COLEQU, EQUIL, NOFACT, NOTRAN, ROWEQU
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|       CHARACTER          NORM
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|       INTEGER            I, INFEQU, J, J1, J2
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|       REAL               AMAX, ANORM, BIGNUM, COLCND, RCMAX, RCMIN,
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|      $                   ROWCND, RPVGRW, SMLNUM
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| *     ..
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| *     .. External Functions ..
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|       LOGICAL            LSAME
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|       REAL               SLAMCH, SLANGB, SLANTB
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|       EXTERNAL           LSAME, SLAMCH, SLANGB, SLANTB
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| *     ..
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| *     .. External Subroutines ..
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|       EXTERNAL           SCOPY, SGBCON, SGBEQU, SGBRFS, SGBTRF, SGBTRS,
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|      $                   SLACPY, SLAQGB, XERBLA
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| *     ..
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| *     .. Intrinsic Functions ..
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|       INTRINSIC          ABS, MAX, MIN
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| *     ..
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| *     .. Executable Statements ..
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| *
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|       INFO = 0
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|       NOFACT = LSAME( FACT, 'N' )
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|       EQUIL = LSAME( FACT, 'E' )
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|       NOTRAN = LSAME( TRANS, 'N' )
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|       IF( NOFACT .OR. EQUIL ) THEN
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|          EQUED = 'N'
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|          ROWEQU = .FALSE.
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|          COLEQU = .FALSE.
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|       ELSE
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|          ROWEQU = LSAME( EQUED, 'R' ) .OR. LSAME( EQUED, 'B' )
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|          COLEQU = LSAME( EQUED, 'C' ) .OR. LSAME( EQUED, 'B' )
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|          SMLNUM = SLAMCH( 'Safe minimum' )
 | |
|          BIGNUM = ONE / SMLNUM
 | |
|       END IF
 | |
| *
 | |
| *     Test the input parameters.
 | |
| *
 | |
|       IF( .NOT.NOFACT .AND. .NOT.EQUIL .AND. .NOT.LSAME( FACT, 'F' ) )
 | |
|      $     THEN
 | |
|          INFO = -1
 | |
|       ELSE IF( .NOT.NOTRAN .AND. .NOT.LSAME( TRANS, 'T' ) .AND. .NOT.
 | |
|      $         LSAME( TRANS, 'C' ) ) THEN
 | |
|          INFO = -2
 | |
|       ELSE IF( N.LT.0 ) THEN
 | |
|          INFO = -3
 | |
|       ELSE IF( KL.LT.0 ) THEN
 | |
|          INFO = -4
 | |
|       ELSE IF( KU.LT.0 ) THEN
 | |
|          INFO = -5
 | |
|       ELSE IF( NRHS.LT.0 ) THEN
 | |
|          INFO = -6
 | |
|       ELSE IF( LDAB.LT.KL+KU+1 ) THEN
 | |
|          INFO = -8
 | |
|       ELSE IF( LDAFB.LT.2*KL+KU+1 ) THEN
 | |
|          INFO = -10
 | |
|       ELSE IF( LSAME( FACT, 'F' ) .AND. .NOT.
 | |
|      $         ( ROWEQU .OR. COLEQU .OR. LSAME( EQUED, 'N' ) ) ) THEN
 | |
|          INFO = -12
 | |
|       ELSE
 | |
|          IF( ROWEQU ) THEN
 | |
|             RCMIN = BIGNUM
 | |
|             RCMAX = ZERO
 | |
|             DO 10 J = 1, N
 | |
|                RCMIN = MIN( RCMIN, R( J ) )
 | |
|                RCMAX = MAX( RCMAX, R( J ) )
 | |
|    10       CONTINUE
 | |
|             IF( RCMIN.LE.ZERO ) THEN
 | |
|                INFO = -13
 | |
|             ELSE IF( N.GT.0 ) THEN
 | |
|                ROWCND = MAX( RCMIN, SMLNUM ) / MIN( RCMAX, BIGNUM )
 | |
|             ELSE
 | |
|                ROWCND = ONE
 | |
|             END IF
 | |
|          END IF
 | |
|          IF( COLEQU .AND. INFO.EQ.0 ) THEN
 | |
|             RCMIN = BIGNUM
 | |
|             RCMAX = ZERO
 | |
|             DO 20 J = 1, N
 | |
|                RCMIN = MIN( RCMIN, C( J ) )
 | |
|                RCMAX = MAX( RCMAX, C( J ) )
 | |
|    20       CONTINUE
 | |
|             IF( RCMIN.LE.ZERO ) THEN
 | |
|                INFO = -14
 | |
|             ELSE IF( N.GT.0 ) THEN
 | |
|                COLCND = MAX( RCMIN, SMLNUM ) / MIN( RCMAX, BIGNUM )
 | |
|             ELSE
 | |
|                COLCND = ONE
 | |
|             END IF
 | |
|          END IF
 | |
|          IF( INFO.EQ.0 ) THEN
 | |
|             IF( LDB.LT.MAX( 1, N ) ) THEN
 | |
|                INFO = -16
 | |
|             ELSE IF( LDX.LT.MAX( 1, N ) ) THEN
 | |
|                INFO = -18
 | |
|             END IF
 | |
|          END IF
 | |
|       END IF
 | |
| *
 | |
|       IF( INFO.NE.0 ) THEN
 | |
|          CALL XERBLA( 'SGBSVX', -INFO )
 | |
|          RETURN
 | |
|       END IF
 | |
| *
 | |
|       IF( EQUIL ) THEN
 | |
| *
 | |
| *        Compute row and column scalings to equilibrate the matrix A.
 | |
| *
 | |
|          CALL SGBEQU( N, N, KL, KU, AB, LDAB, R, C, ROWCND, COLCND,
 | |
|      $                AMAX, INFEQU )
 | |
|          IF( INFEQU.EQ.0 ) THEN
 | |
| *
 | |
| *           Equilibrate the matrix.
 | |
| *
 | |
|             CALL SLAQGB( N, N, KL, KU, AB, LDAB, R, C, ROWCND, COLCND,
 | |
|      $                   AMAX, EQUED )
 | |
|             ROWEQU = LSAME( EQUED, 'R' ) .OR. LSAME( EQUED, 'B' )
 | |
|             COLEQU = LSAME( EQUED, 'C' ) .OR. LSAME( EQUED, 'B' )
 | |
|          END IF
 | |
|       END IF
 | |
| *
 | |
| *     Scale the right hand side.
 | |
| *
 | |
|       IF( NOTRAN ) THEN
 | |
|          IF( ROWEQU ) THEN
 | |
|             DO 40 J = 1, NRHS
 | |
|                DO 30 I = 1, N
 | |
|                   B( I, J ) = R( I )*B( I, J )
 | |
|    30          CONTINUE
 | |
|    40       CONTINUE
 | |
|          END IF
 | |
|       ELSE IF( COLEQU ) THEN
 | |
|          DO 60 J = 1, NRHS
 | |
|             DO 50 I = 1, N
 | |
|                B( I, J ) = C( I )*B( I, J )
 | |
|    50       CONTINUE
 | |
|    60    CONTINUE
 | |
|       END IF
 | |
| *
 | |
|       IF( NOFACT .OR. EQUIL ) THEN
 | |
| *
 | |
| *        Compute the LU factorization of the band matrix A.
 | |
| *
 | |
|          DO 70 J = 1, N
 | |
|             J1 = MAX( J-KU, 1 )
 | |
|             J2 = MIN( J+KL, N )
 | |
|             CALL SCOPY( J2-J1+1, AB( KU+1-J+J1, J ), 1,
 | |
|      $                  AFB( KL+KU+1-J+J1, J ), 1 )
 | |
|    70    CONTINUE
 | |
| *
 | |
|          CALL SGBTRF( N, N, KL, KU, AFB, LDAFB, IPIV, INFO )
 | |
| *
 | |
| *        Return if INFO is non-zero.
 | |
| *
 | |
|          IF( INFO.GT.0 ) THEN
 | |
| *
 | |
| *           Compute the reciprocal pivot growth factor of the
 | |
| *           leading rank-deficient INFO columns of A.
 | |
| *
 | |
|             ANORM = ZERO
 | |
|             DO 90 J = 1, INFO
 | |
|                DO 80 I = MAX( KU+2-J, 1 ), MIN( N+KU+1-J, KL+KU+1 )
 | |
|                   ANORM = MAX( ANORM, ABS( AB( I, J ) ) )
 | |
|    80          CONTINUE
 | |
|    90       CONTINUE
 | |
|             RPVGRW = SLANTB( 'M', 'U', 'N', INFO, MIN( INFO-1, KL+KU ),
 | |
|      $                       AFB( MAX( 1, KL+KU+2-INFO ), 1 ), LDAFB,
 | |
|      $                       WORK )
 | |
|             IF( RPVGRW.EQ.ZERO ) THEN
 | |
|                RPVGRW = ONE
 | |
|             ELSE
 | |
|                RPVGRW = ANORM / RPVGRW
 | |
|             END IF
 | |
|             WORK( 1 ) = RPVGRW
 | |
|             RCOND = ZERO
 | |
|             RETURN
 | |
|          END IF
 | |
|       END IF
 | |
| *
 | |
| *     Compute the norm of the matrix A and the
 | |
| *     reciprocal pivot growth factor RPVGRW.
 | |
| *
 | |
|       IF( NOTRAN ) THEN
 | |
|          NORM = '1'
 | |
|       ELSE
 | |
|          NORM = 'I'
 | |
|       END IF
 | |
|       ANORM = SLANGB( NORM, N, KL, KU, AB, LDAB, WORK )
 | |
|       RPVGRW = SLANTB( 'M', 'U', 'N', N, KL+KU, AFB, LDAFB, WORK )
 | |
|       IF( RPVGRW.EQ.ZERO ) THEN
 | |
|          RPVGRW = ONE
 | |
|       ELSE
 | |
|          RPVGRW = SLANGB( 'M', N, KL, KU, AB, LDAB, WORK ) / RPVGRW
 | |
|       END IF
 | |
| *
 | |
| *     Compute the reciprocal of the condition number of A.
 | |
| *
 | |
|       CALL SGBCON( NORM, N, KL, KU, AFB, LDAFB, IPIV, ANORM, RCOND,
 | |
|      $             WORK, IWORK, INFO )
 | |
| *
 | |
| *     Compute the solution matrix X.
 | |
| *
 | |
|       CALL SLACPY( 'Full', N, NRHS, B, LDB, X, LDX )
 | |
|       CALL SGBTRS( TRANS, N, KL, KU, NRHS, AFB, LDAFB, IPIV, X, LDX,
 | |
|      $             INFO )
 | |
| *
 | |
| *     Use iterative refinement to improve the computed solution and
 | |
| *     compute error bounds and backward error estimates for it.
 | |
| *
 | |
|       CALL SGBRFS( TRANS, N, KL, KU, NRHS, AB, LDAB, AFB, LDAFB, IPIV,
 | |
|      $             B, LDB, X, LDX, FERR, BERR, WORK, IWORK, INFO )
 | |
| *
 | |
| *     Transform the solution matrix X to a solution of the original
 | |
| *     system.
 | |
| *
 | |
|       IF( NOTRAN ) THEN
 | |
|          IF( COLEQU ) THEN
 | |
|             DO 110 J = 1, NRHS
 | |
|                DO 100 I = 1, N
 | |
|                   X( I, J ) = C( I )*X( I, J )
 | |
|   100          CONTINUE
 | |
|   110       CONTINUE
 | |
|             DO 120 J = 1, NRHS
 | |
|                FERR( J ) = FERR( J ) / COLCND
 | |
|   120       CONTINUE
 | |
|          END IF
 | |
|       ELSE IF( ROWEQU ) THEN
 | |
|          DO 140 J = 1, NRHS
 | |
|             DO 130 I = 1, N
 | |
|                X( I, J ) = R( I )*X( I, J )
 | |
|   130       CONTINUE
 | |
|   140    CONTINUE
 | |
|          DO 150 J = 1, NRHS
 | |
|             FERR( J ) = FERR( J ) / ROWCND
 | |
|   150    CONTINUE
 | |
|       END IF
 | |
| *
 | |
| *     Set INFO = N+1 if the matrix is singular to working precision.
 | |
| *
 | |
|       IF( RCOND.LT.SLAMCH( 'Epsilon' ) )
 | |
|      $   INFO = N + 1
 | |
| *
 | |
|       WORK( 1 ) = RPVGRW
 | |
|       RETURN
 | |
| *
 | |
| *     End of SGBSVX
 | |
| *
 | |
|       END
 |