DSPSVX(1) LAPACK driver routine (version 3.2) DSPSVX(1)NAME
DSPSVX - uses the diagonal pivoting factorization A = U*D*U**T or A =
L*D*L**T to compute the solution to a real system of linear equations A
* X = B, where A is an N-by-N symmetric matrix stored in packed format
and X and B are N-by-NRHS matrices
SYNOPSIS
SUBROUTINE DSPSVX( FACT, UPLO, N, NRHS, AP, AFP, IPIV, B, LDB, X, LDX,
RCOND, FERR, BERR, WORK, IWORK, INFO )
CHARACTER FACT, UPLO
INTEGER INFO, LDB, LDX, N, NRHS
DOUBLE PRECISION RCOND
INTEGER IPIV( * ), IWORK( * )
DOUBLE PRECISION AFP( * ), AP( * ), B( LDB, * ), BERR( * ),
FERR( * ), WORK( * ), X( LDX, * )
PURPOSE
DSPSVX uses the diagonal pivoting factorization A = U*D*U**T or A =
L*D*L**T to compute the solution to a real system of linear equations A
* X = B, where A is an N-by-N symmetric matrix stored in packed format
and X and B are N-by-NRHS matrices. Error bounds on the solution and a
condition estimate are also provided.
DESCRIPTION
The following steps are performed:
1. If FACT = 'N', the diagonal pivoting method is used to factor A as
A = U * D * U**T, if UPLO = 'U', or
A = L * D * L**T, if UPLO = 'L',
where U (or L) is a product of permutation and unit upper (lower)
triangular matrices and D is symmetric and block diagonal with
1-by-1 and 2-by-2 diagonal blocks.
2. If some D(i,i)=0, so that D 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. 3. The
system of equations is solved for X using the factored form
of A.
4. Iterative refinement is applied to improve the computed solution
matrix and calculate error bounds and backward error estimates
for it.
ARGUMENTS
FACT (input) CHARACTER*1
Specifies whether or not the factored form of A has been sup‐
plied on entry. = 'F': On entry, AFP and IPIV contain the
factored form of A. AP, AFP and IPIV will not be modified. =
'N': The matrix A will be copied to AFP and factored.
UPLO (input) CHARACTER*1
= 'U': Upper triangle of A is stored;
= 'L': Lower triangle of A is stored.
N (input) INTEGER
The number of linear equations, i.e., the order 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.
AP (input) DOUBLE PRECISION array, dimension (N*(N+1)/2)
The upper or lower triangle of the symmetric matrix A, packed
columnwise in a linear array. The j-th column of A is stored
in the array AP as follows: if UPLO = 'U', AP(i + (j-1)*j/2) =
A(i,j) for 1<=i<=j; if UPLO = 'L', AP(i + (j-1)*(2*n-j)/2) =
A(i,j) for j<=i<=n. See below for further details.
AFP (input or output) DOUBLE PRECISION array, dimension
(N*(N+1)/2) If FACT = 'F', then AFP is an input argument and on
entry contains the block diagonal matrix D and the multipliers
used to obtain the factor U or L from the factorization A =
U*D*U**T or A = L*D*L**T as computed by DSPTRF, stored as a
packed triangular matrix in the same storage format as A. If
FACT = 'N', then AFP is an output argument and on exit contains
the block diagonal matrix D and the multipliers used to obtain
the factor U or L from the factorization A = U*D*U**T or A =
L*D*L**T as computed by DSPTRF, stored as a packed triangular
matrix in the same storage format as A.
IPIV (input or output) INTEGER array, dimension (N)
If FACT = 'F', then IPIV is an input argument and on entry con‐
tains details of the interchanges and the block structure of D,
as determined by DSPTRF. If IPIV(k) > 0, then rows and columns
k and IPIV(k) were interchanged and D(k,k) is a 1-by-1 diagonal
block. If UPLO = 'U' and IPIV(k) = IPIV(k-1) < 0, then rows
and columns k-1 and -IPIV(k) were interchanged and
D(k-1:k,k-1:k) is a 2-by-2 diagonal block. If UPLO = 'L' and
IPIV(k) = IPIV(k+1) < 0, then rows and columns k+1 and -IPIV(k)
were interchanged and D(k:k+1,k:k+1) is a 2-by-2 diagonal
block. If FACT = 'N', then IPIV is an output argument and on
exit contains details of the interchanges and the block struc‐
ture of D, as determined by DSPTRF.
B (input) DOUBLE PRECISION array, dimension (LDB,NRHS)
The N-by-NRHS right hand side matrix B.
LDB (input) INTEGER
The leading dimension of the array B. LDB >= max(1,N).
X (output) DOUBLE PRECISION array, dimension (LDX,NRHS)
If INFO = 0 or INFO = N+1, the N-by-NRHS solution matrix X.
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. 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) DOUBLE PRECISION array, dimension (3*N)
IWORK (workspace) INTEGER array, dimension (N)
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: D(i,i) is exactly zero. The factorization has been com‐
pleted but the factor D is exactly singular, so the solution
and error bounds could not be computed. RCOND = 0 is returned.
= N+1: D 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.
FURTHER DETAILS
The packed storage scheme is illustrated by the following example when
N = 4, UPLO = 'U':
Two-dimensional storage of the symmetric matrix A:
a11 a12 a13 a14
a22 a23 a24
a33 a34 (aij = aji)
a44
Packed storage of the upper triangle of A:
AP = [ a11, a12, a22, a13, a23, a33, a14, a24, a34, a44 ]
LAPACK driver routine (version 3November 2008 DSPSVX(1)
