# p?trti2

Computes the inverse of a triangular matrix (local unblocked algorithm).

## Syntax

call pstrti2(uplo, diag, n, a, ia, ja, desca, info)

call pdtrti2(uplo, diag, n, a, ia, ja, desca, info)

call pctrti2(uplo, diag, n, a, ia, ja, desca, info)

call pztrti2(uplo, diag, n, a, ia, ja, desca, info)

## Description

This routine computes the inverse of a real/complex upper or lower triangular block matrix sub `(A) = A(ia:ia+n-1`, `ja:ja+n-1)`.

This matrix should be contained in one and only one process memory space (local operation).

## Input Parameters

uplo

  (global) CHARACTER*1.

  Specifies whether the matrix sub (A) is upper or lower triangular.

  = 'U': sub (A) is upper triangular

  = 'L': sub (A) is lower triangular.

diag

  (global) CHARACTER*1.

  Specifies whether or not the matrix A is unit triangular.

  = 'N': sub (A) is non-unit triangular

  = 'U': sub (A) is unit triangular.

n

  (global) INTEGER.

  The number of rows and columns to be operated on, i.e., the order of the distributed submatrix `sub(A)`. `n ≥ 0`.

a

  (local)

  REAL for pstrti2

  DOUBLE PRECISION for pdtrti2

  COMPLEX for pctrti2

  COMPLEX*16 for pztrti2.

  Pointer into the local memory to an array, `DIMENSION`(lld_a`, LOCc(ja+n-1))`.

  On entry, this array contains the local pieces of the triangular matrix sub(A).

  If `uplo = 'U'`, the leading n-by-n upper triangular part of the matrix sub(A) contains the upper triangular part of the matrix, and the strictly lower triangular part of sub(A) is not referenced.

  If `uplo = 'L'`, the leading n-by-n lower triangular part of the matrix sub(A) contains the lower triangular part of the matrix, and the strictly upper triangular part of sub(A) is not referenced. If `diag = 'U'`, the diagonal elements of sub(A) are not referenced either and are assumed to be 1.

ia, ja

  (global) INTEGER.

  The row and column indices in the global array A indicating the first row and the first column of the sub(A), respectively.

desca

  (global and local) INTEGER array, DIMENSION (dlen_). The array descriptor for the distributed matrix A.

## Output Parameters

a

  On exit, the (triangular) inverse of the original matrix, in the same storage format.

info

  INTEGER.

  `= 0`: successful exit

  `< 0`: if the i-th argument is an array and the j-entry had an illegal value,

  then `info = - (i*100)`,

  if the i-th argument is a scalar and had an illegal value,

  then `info = -i`.

Parent topic: Auxiliary Routines