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# zhpsvx

```
NAME
ZHPSVX - use the diagonal pivoting factorization A =
U*D*U**H or A = L*D*L**H to compute the solution to a com-
plex system of linear equations A * X = B, where A is an N-
by-N Hermitian matrix stored in packed format and X and B
are N-by-NRHS matrices

SYNOPSIS
SUBROUTINE ZHPSVX( FACT, UPLO, N, NRHS, AP, AFP, IPIV, B,
LDB, X, LDX, RCOND, FERR, BERR, WORK,
RWORK, INFO )

CHARACTER      FACT, UPLO

INTEGER        INFO, LDB, LDX, N, NRHS

DOUBLE         PRECISION RCOND

INTEGER        IPIV( * )

DOUBLE         PRECISION BERR( * ), FERR( * ), RWORK( *
)

COMPLEX*16     AFP( * ), AP( * ), B( LDB, * ), WORK( *
), X( LDX, * )

PURPOSE
ZHPSVX uses the diagonal pivoting factorization A = U*D*U**H
or A = L*D*L**H to compute the solution to a complex system
of linear equations A * X = B, where A is an N-by-N Hermi-
tian 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**H,  if UPLO = 'U', or
A = L * D * L**H,  if UPLO = 'L',
where U (or L) is a product of permutation and unit upper
(lower)
triangular matrices and D is Hermitian and block diagonal
with
1-by-1 and 2-by-2 diagonal blocks.

2. 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, steps 3 and 4 are skipped.

3. The system of equations is solved for X using the fac-
tored 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 supplied on entry.  = 'F':  On entry, AFP and
IPIV contain the factored form of A.  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) COMPLEX*16 array, dimension (N*(N+1)/2)
The upper or lower triangle of the Hermitian 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)*(2n-j)/2) = A(i,j) for
j<=i<=n.  See below for further details.

AFP     (input or output) COMPLEX*16 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**H or A = L*D*L**H as
computed by ZHPTRF, 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**H or A = L*D*L**H as
computed by ZHPTRF, 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 contains details of the interchanges and the
block structure of D, as determined by ZHPTRF.  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 inter-
changed 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 inter-
changed 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 structure of D, as determined by ZHPTRF.

B       (input) COMPLEX*16 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) COMPLEX*16 array, dimension (LDX,NRHS)
If INFO = 0, 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, and the
solution and error bounds are not computed.

FERR    (output) DOUBLE PRECISION array, dimension (NRHS)
The estimated forward error bounds for each solution
vector X(j) (the j-th column of the solution matrix
X).  If XTRUE is the true solution, FERR(j) bounds
the magnitude of the largest entry in (X(j) - XTRUE)
divided by the magnitude of the largest entry in

X(j).  The quality of the error bound depends on the
quality of the estimate of norm(inv(A)) computed in
the code; if the estimate of norm(inv(A)) is accu-
rate, the error bound is guaranteed.

BERR    (output) DOUBLE PRECISION array, dimension (NRHS)
The componentwise relative backward error of each
solution vector X(j) (i.e., the smallest relative
change in any entry of A or B that makes X(j) an
exact solution).

WORK    (workspace) COMPLEX*16 array, dimension (2*N)

RWORK   (workspace) DOUBLE PRECISION array, dimension (N)

INFO    (output) INTEGER
= 0: successful exit
< 0: if INFO = -i, the i-th argument had an illegal
value
> 0 and <= N: if INFO = i, D(i,i) is exactly zero.
The factorization has been completed, but the block
diagonal matrix D is exactly singular, so the solu-
tion and error bounds could not be computed.  = N+1:
the block diagonal matrix D is nonsingular, but
RCOND is less than machine precision.  The factori-
zation has been completed, but the matrix is singu-
lar to working precision, so the solution and error
bounds have not been computed.

FURTHER DETAILS
The packed storage scheme is illustrated by the following
example when N = 4, UPLO = 'U':

Two-dimensional storage of the Hermitian matrix A:

a11 a12 a13 a14
a22 a23 a24
a33 a34     (aij = conjg(aji))
a44

Packed storage of the upper triangle of A:

AP = [ a11, a12, a22, a13, a23, a33, a14, a24, a34, a44 ]
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