STPRFS(3)
provides error bounds and backward error estimates for the solution to a system of linear equations with a triangular packed coefficient matrix
SYNOPSIS
 SUBROUTINE STPRFS(

UPLO, TRANS, DIAG, N, NRHS, AP, B, LDB, X, LDX,
FERR, BERR, WORK, IWORK, INFO )

CHARACTER
DIAG, TRANS, UPLO

INTEGER
INFO, LDB, LDX, N, NRHS

INTEGER
IWORK( * )

REAL
AP( * ), B( LDB, * ), BERR( * ), FERR( * ),
WORK( * ), X( LDX, * )
PURPOSE
STPRFS provides error bounds and backward error estimates for the
solution to a system of linear equations with a triangular packed
coefficient matrix.
The solution matrix X must be computed by STPTRS or some other
means before entering this routine. STPRFS does not do iterative
refinement because doing so cannot improve the backward error.
ARGUMENTS
 UPLO (input) CHARACTER*1

= 'U': A is upper triangular;
= 'L': A is lower triangular.
 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 = Transpose)
 DIAG (input) CHARACTER*1

= 'N': A is nonunit triangular;
= 'U': A is unit triangular.
 N (input) INTEGER

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) REAL array, dimension (N*(N+1)/2)

The upper or lower triangular matrix A, packed columnwise in
a linear array. The jth column of A is stored in the array
AP as follows:
if UPLO = 'U', AP(i + (j1)*j/2) = A(i,j) for 1<=i<=j;
if UPLO = 'L', AP(i + (j1)*(2*nj)/2) = A(i,j) for j<=i<=n.
If DIAG = 'U', the diagonal elements of A are not referenced
and are assumed to be 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) REAL array, dimension (LDX,NRHS)

The 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