** Next:** Shift and invert on
** Up:** General Use of ARPACK
** Previous:** is NOT Hermitian Positive

The reverse communication interface routine for all problem types
is `__aupd`. If the eigenvalue problem (3.2.1)
is a double precision non-symmetric one,
then the subroutine to use is `dnaupd`.
First the reverse communication loop structure will be described
and then the details and nuances of the problem set up will
be discussed.
We shall use the symbol `OP` for the
operator that is applied to vectors in the Arnoldi/Lanczos process
and will stand for the matrix to use in the
weighted inner product described previously.
For the shift-invert spectral transformation mode,
`OP` denotes and denotes . They will stand for different
matrices in each of the various modes.
The basic idea is to set up a loop that repeatedly calls `__aupd`.
On each return, one must
either apply `OP` or to a specified vector or exit the loop
depending upon the value returned in the reverse communication
parameter `ido`.

*Chao Yang*

*11/7/1997*