---
name: "stptri"
section: "3lapack"
platform: "linux"
package: "liblapack-doc"
version: "3.2.1-8"
release: "Debian 6.0 (Squeeze)"
platforms: ["linux"]
url: "https://new.carta.tech/stptri/3lapack"
---

# stptri(3lapack)

computes the inverse of a real upper or lower triangular matrix A stored in packed format

## NAME

STPTRI - computes the inverse of a real upper or lower triangular matrix A stored in packed format

## SYNOPSIS

- SUBROUTINE STPTRI(

  UPLO, DIAG, N, AP, INFO )

- CHARACTER

  DIAG, UPLO

- INTEGER

  INFO, N

- REAL

  AP( \* )

## PURPOSE

STPTRI computes the inverse of a real upper or lower triangular matrix A stored in packed format.

## 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.

- AP (input/output) REAL array, dimension (N\*(N+1)/2)

  On entry, the upper or lower triangular matrix A, stored 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. See below for further details. On exit, the (triangular) inverse of the original matrix, in the same packed storage format.

- 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 triangular matrix is singular and its inverse can not be computed.

## FURTHER DETAILS

A triangular matrix A can be transferred to packed storage using one of the following program segments:\
 UPLO = 'U':                      UPLO = 'L':\
\
       JC = 1                           JC = 1\
\
       DO 2 J = 1, N                    DO 2 J = 1, N\
\
          DO 1 I = 1, J                    DO 1 I = J, N\
\
             AP(JC+I-1) = A(I,J)              AP(JC+I-J) = A(I,J)\
     1    CONTINUE                    1    CONTINUE\
\
          JC = JC + J                      JC = JC + N - J + 1\
     2 CONTINUE                       2 CONTINUE\