STPTRI computes the inverse of a real upper or lower triangular


 matrix A stored in packed format.

UPLO

          UPLO is CHARACTER*1


          = 'U':  A is upper triangular;


          = 'L':  A is lower triangular.

DIAG

          DIAG is CHARACTER*1


          = 'N':  A is non-unit triangular;


          = 'U':  A is unit triangular.

N

          N is INTEGER


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

AP

          AP is 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

          INFO is 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.

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

December 2016

  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