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# strtri

```
NAME
STRTRI - compute the inverse of a real upper or lower tri-
angular matrix A

SYNOPSIS
SUBROUTINE STRTRI( UPLO, DIAG, N, A, LDA, INFO )

CHARACTER      DIAG, UPLO

INTEGER        INFO, LDA, N

REAL           A( LDA, * )

PURPOSE
STRTRI computes the inverse of a real upper or lower tri-
angular matrix A.

This is the Level 3 BLAS version of the algorithm.

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

DIAG    (input) CHARACTER*1
= 'N':  A is non-unit triangular;
= 'U':  A is unit triangular.

N       (input) INTEGER
The order of the matrix A.  N >= 0.

A       (input/output) REAL array, dimension (LDA,N)
On entry, the triangular matrix A.  If UPLO = 'U',
the leading N-by-N upper triangular part of the
array A contains the upper triangular matrix, and
the strictly lower triangular part of A is not
referenced.  If UPLO = 'L', the leading N-by-N lower
triangular part of the array A contains the lower
triangular matrix, and the strictly upper triangular
part of A is not referenced.  If DIAG = 'U', the
diagonal elements of A are also not referenced and
are assumed to be 1.  On exit, the (triangular)
inverse of the original matrix, in the same storage
format.

LDA     (input) INTEGER
The leading dimension of the array A.  LDA >=
max(1,N).

INFO    (output) INTEGER
= 0: successful exit

< 0: if INFO = -i, the i-th argument had an illegal
value
> 0: if INFO = i, A(i,i) is exactly zero.  The tri-
angular matrix is singular and its inverse can not
be computed.
```