SPRFS - 5.2 English - 68552

SPRFS improves the computed solution to a system of linear equations

when the coefficient matrix is symmetric indefinite and packed.

Purpose:

  SPRFS improves the computed solution to a system of linear
  equations when the coefficient matrix is symmetric indefinite
  and packed, and provides error bounds and backward error estimates
  for the solution.

Parameters:

• UPLO – [in]

  UPLO is CHARACTER*1

  = ‘U’: Upper triangle of A is stored;

  = ‘L’: Lower triangle of A is stored.

• N – [in]

  N is INTEGER

  The order of the matrix A. N >= 0.

• NRHS – [in]

  NRHS is INTEGER

  The number of right hand sides, i.e., the number of columns of the matrices B and X. NRHS >= 0.

• AP – [in]

  AP is REAL 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.

• AFP – [in]

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

  The factored form of the matrix A. AFP 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 SSPTRF, stored as a packed triangular matrix.

• IPIV – [in]

  IPIV is INTEGER array, dimension (N)

  Details of the interchanges and the block structure of D as determined by SSPTRF.

• B – [in]

  B is REAL array, dimension (LDB,NRHS)

  The right hand side matrix B.

• LDB – [in]

  LDB is INTEGER

  The leading dimension of the array B. LDB >= fla_max(1,N).

• X – [inout]

  X is REAL array, dimension (LDX,NRHS)

  On entry, the solution matrix X, as computed by SSPTRS. On exit, the improved solution matrix X.

• LDX – [in]

  LDX is INTEGER

  The leading dimension of the array X. LDX >= fla_max(1,N).

• FERR – [out]

  FERR is REAL 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 – [out]

  BERR is 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 – [out] WORK is REAL array, dimension (3*N)

• IWORK – [out] IWORK is INTEGER array, dimension (N)

• INFO – [out]

  INFO is INTEGER

  = 0: successful exit

  < 0: if INFO = -i, the i-th argument had an illegal value