DSYTRI2 computes the inverse of a DOUBLE PRECISION symmetric indefinite matrix


 A using the factorization A = U*D*U**T or A = L*D*L**T computed by


 DSYTRF. DSYTRI2 sets the LEADING DIMENSION of the workspace


 before calling DSYTRI2X that actually computes the inverse.

UPLO

          UPLO is 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

          N is INTEGER


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

A

          A is DOUBLE PRECISION array, dimension (LDA,N)


          On entry, the NB diagonal matrix D and the multipliers


          used to obtain the factor U or L as computed by DSYTRF.


          On exit, if INFO = 0, the (symmetric) inverse of the original


          matrix.  If UPLO = 'U', the upper triangular part of the


          inverse is formed and the part of A below the diagonal is not


          referenced; if UPLO = 'L' the lower triangular part of the


          inverse is formed and the part of A above the diagonal is


          not referenced.

LDA

          LDA is INTEGER


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

IPIV

          IPIV is INTEGER array, dimension (N)


          Details of the interchanges and the NB structure of D


          as determined by DSYTRF.

WORK

          WORK is DOUBLE PRECISION array, dimension (N+NB+1)*(NB+3)

LWORK

          LWORK is INTEGER


          The dimension of the array WORK.


          WORK is size >= (N+NB+1)*(NB+3)


          If LDWORK = -1, then a workspace query is assumed; the routine


           calculates:


              - the optimal size of the WORK array, returns


          this value as the first entry of the WORK array,


              - and no error message related to LDWORK is issued by XERBLA.

INFO

          INFO is INTEGER


          = 0: successful exit


          < 0: if INFO = -i, the i-th argument had an illegal value


          > 0: if INFO = i, D(i,i) = 0; the matrix is singular and its


               inverse could not be computed.

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

November 2017