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Driver `dsdrv4` uses the shift-inverse mode to solve the
generalized eigenvalue problem. Eigenvalues closest to a shift
can be obtained by computing selected eigenvalues
for
To compute the eigenvalues of () just to the right of , one should set `which = 'LA'` and for those just to the left
set `which = 'SA'`. Eigenvalues closest to may
be obtained by setting `which = 'BE'`.
The eigenvalue of the original problem and are
related by

To use `dsdrv4`, the user is required to supply the two matrix vector
operations
where is the shift defined by the user.
Typically, the above matrix operation is performed by factoring
once at the beginning and and
then using this factorization repeatedly to solve the resulting linear
system. Note, this will require a symmetric *indefinite*
factorization whenever is a point in the interior of the
spectrum of (). A general (sparse) *LU * factorization may
also be used, but there will be a storage penalty for ignoring symmetry.

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** Up:** Selecting a Symmetric Driver
** Previous:** Regular Inverse Mode
*Chao Yang*

*11/7/1997*