- SUBROUTINE CTPTRI(
- UPLO, DIAG, N, AP, INFO )
- CHARACTER DIAG, UPLO
- INTEGER INFO, N
- COMPLEX AP( * )
PURPOSECTPTRI computes the inverse of a complex upper or lower triangular matrix A stored in packed format.
- 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) COMPLEX 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 DETAILSA 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