SSPTRI  -  compute the inverse of a real symmetric indefi­
       nite matrix A in packed storage using the factorization  A
       = U*D*U**T or A = L*D*L**T computed by SSPTRF


SYNOPSIS

       SUBROUTINE SSPTRI( UPLO, N, AP, IPIV, WORK, INFO )

           CHARACTER      UPLO

           INTEGER        INFO, N

           INTEGER        IPIV( * )

           REAL           AP( * ), WORK( * )


PURPOSE

       SSPTRI computes the inverse of a real symmetric indefinite
       matrix A in packed storage using  the  factorization  A  =
       U*D*U**T or A = L*D*L**T computed by SSPTRF.


ARGUMENTS

       UPLO    (input) CHARACTER*1
               Specifies whether the details of the factorization
               are stored as an upper or lower triangular matrix.
               = 'U':  Upper triangular, form is A = U*D*U**T;
               = 'L':  Lower triangular, form is A = L*D*L**T.

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

       AP      (input/output) REAL array, dimension (N*(N+1)/2)
               On entry, the block diagonal matrix D and the mul­
               tipliers used to obtain the factor U or L as  com­
               puted  by  SSPTRF,  stored  as a packed triangular
               matrix.

               On exit, if INFO = 0, the (symmetric)  inverse  of
               the original matrix, stored as a packed triangular
               matrix. The j-th column of inv(A) is stored in the
               array  AP  as  follows:  if  UPLO  =  'U',  AP(i +
               (j-1)*j/2) = inv(A)(i,j) for 1<=i<=j;  if  UPLO  =
               'L',  AP(i  +  (j-1)*(2n-j)/2)  =  inv(A)(i,j) for
               j<=i<=n.

       IPIV    (input) INTEGER array, dimension (N)
               Details of the interchanges and the  block  struc­
               ture of D as determined by SSPTRF.

       WORK    (workspace) REAL array, dimension (N)

       INFO    (output) INTEGER
               = 0: successful exit
               <  0:  if  INFO  =  -i,  the  i-th argument had an

               > 0: if INFO = i, D(i,i) = 0; the matrix is singu­
               lar and its inverse could not be computed.


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