SPBRFS(3F) SPBRFS(3F)
SPBRFS  improve the computed solution to a system of linear equations
when the coefficient matrix is symmetric positive definite and banded,
and provides error bounds and backward error estimates for the solution
SUBROUTINE SPBRFS( UPLO, N, KD, NRHS, AB, LDAB, AFB, LDAFB, B, LDB, X,
LDX, FERR, BERR, WORK, IWORK, INFO )
CHARACTER UPLO
INTEGER INFO, KD, LDAB, LDAFB, LDB, LDX, N, NRHS
INTEGER IWORK( * )
REAL AB( LDAB, * ), AFB( LDAFB, * ), B( LDB, * ), BERR( *
), FERR( * ), WORK( * ), X( LDX, * )
SPBRFS improves the computed solution to a system of linear equations
when the coefficient matrix is symmetric positive definite and banded,
and provides error bounds and backward error estimates for the solution.
UPLO (input) CHARACTER*1
= 'U': Upper triangle of A is stored;
= 'L': Lower triangle of A is stored.
N (input) INTEGER
The order of the matrix A. N >= 0.
KD (input) INTEGER
The number of superdiagonals of the matrix A if UPLO = 'U', or
the number of subdiagonals if UPLO = 'L'. KD >= 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) REAL array, dimension (LDAB,N)
The upper or lower triangle of the symmetric band matrix A,
stored in the first KD+1 rows of the array. The jth column of A
is stored in the jth column of the array AB as follows: if UPLO
= 'U', AB(kd+1+ij,j) = A(i,j) for max(1,jkd)<=i<=j; if UPLO =
'L', AB(1+ij,j) = A(i,j) for j<=i<=min(n,j+kd).
LDAB (input) INTEGER
The leading dimension of the array AB. LDAB >= KD+1.
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SPBRFS(3F) SPBRFS(3F)
AFB (input) REAL array, dimension (LDAFB,N)
The triangular factor U or L from the Cholesky factorization A =
U**T*U or A = L*L**T of the band matrix A as computed by SPBTRF,
in the same storage format as A (see AB).
LDAFB (input) INTEGER
The leading dimension of the array AFB. LDAFB >= KD+1.
B (input) REAL array, dimension (LDB,NRHS)
The right hand side matrix B.
LDB (input) INTEGER
The leading dimension of the array B. LDB >= max(1,N).
X (input/output) REAL array, dimension (LDX,NRHS)
On entry, the solution matrix X, as computed by SPBTRS. On exit,
the improved solution matrix X.
LDX (input) INTEGER
The leading dimension of the array X. LDX >= max(1,N).
FERR (output) REAL array, dimension (NRHS)
The estimated forward error bound for each solution vector X(j)
(the jth 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) REAL array, dimension (NRHS)
The componentwise relative backward error of each solution vector
X(j) (i.e., the smallest relative change in any element of A or B
that makes X(j) an exact solution).
WORK (workspace) REAL array, dimension (3*N)
IWORK (workspace) INTEGER array, dimension (N)
INFO (output) INTEGER
= 0: successful exit
< 0: if INFO = i, the ith argument had an illegal value
ITMAX is the maximum number of steps of iterative refinement.
SPBRFS(3F) SPBRFS(3F)
SPBRFS  improve the computed solution to a system of linear equations
when the coefficient matrix is symmetric positive definite and banded,
and provides error bounds and backward error estimates for the solution
SUBROUTINE SPBRFS( UPLO, N, KD, NRHS, AB, LDAB, AFB, LDAFB, B, LDB, X,
LDX, FERR, BERR, WORK, IWORK, INFO )
CHARACTER UPLO
INTEGER INFO, KD, LDAB, LDAFB, LDB, LDX, N, NRHS
INTEGER IWORK( * )
REAL AB( LDAB, * ), AFB( LDAFB, * ), B( LDB, * ), BERR( *
), FERR( * ), WORK( * ), X( LDX, * )
SPBRFS improves the computed solution to a system of linear equations
when the coefficient matrix is symmetric positive definite and banded,
and provides error bounds and backward error estimates for the solution.
UPLO (input) CHARACTER*1
= 'U': Upper triangle of A is stored;
= 'L': Lower triangle of A is stored.
N (input) INTEGER
The order of the matrix A. N >= 0.
KD (input) INTEGER
The number of superdiagonals of the matrix A if UPLO = 'U', or
the number of subdiagonals if UPLO = 'L'. KD >= 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) REAL array, dimension (LDAB,N)
The upper or lower triangle of the symmetric band matrix A,
stored in the first KD+1 rows of the array. The jth column of A
is stored in the jth column of the array AB as follows: if UPLO
= 'U', AB(kd+1+ij,j) = A(i,j) for max(1,jkd)<=i<=j; if UPLO =
'L', AB(1+ij,j) = A(i,j) for j<=i<=min(n,j+kd).
LDAB (input) INTEGER
The leading dimension of the array AB. LDAB >= KD+1.
Page 1
SPBRFS(3F) SPBRFS(3F)
AFB (input) REAL array, dimension (LDAFB,N)
The triangular factor U or L from the Cholesky factorization A =
U**T*U or A = L*L**T of the band matrix A as computed by SPBTRF,
in the same storage format as A (see AB).
LDAFB (input) INTEGER
The leading dimension of the array AFB. LDAFB >= KD+1.
B (input) REAL array, dimension (LDB,NRHS)
The right hand side matrix B.
LDB (input) INTEGER
The leading dimension of the array B. LDB >= max(1,N).
X (input/output) REAL array, dimension (LDX,NRHS)
On entry, the solution matrix X, as computed by SPBTRS. On exit,
the improved solution matrix X.
LDX (input) INTEGER
The leading dimension of the array X. LDX >= max(1,N).
FERR (output) REAL array, dimension (NRHS)
The estimated forward error bound for each solution vector X(j)
(the jth 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) REAL array, dimension (NRHS)
The componentwise relative backward error of each solution vector
X(j) (i.e., the smallest relative change in any element of A or B
that makes X(j) an exact solution).
WORK (workspace) REAL array, dimension (3*N)
IWORK (workspace) INTEGER array, dimension (N)
INFO (output) INTEGER
= 0: successful exit
< 0: if INFO = i, the ith argument had an illegal value
ITMAX is the maximum number of steps of iterative refinement.
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