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Driver `dndrv2` uses the shift-invert mode to find
eigenvalues closest to a real shift This is often used
to compute interior eigenvalues. For a discussion of shift-invert mode,
see 3.2 in Chapter 3. To use `dndrv2`,
the user is required to supply the action of
This is typically accomplished by factoring the matrix once before the iteration begins and then using this factorization
repeatedly to solve the sequence of linear systems that arise during
the calculation. The IRAM will find selected
eigenvalues of depending on the
setting of `which`. The recommended setting for computing
the eigenvalues of *A* nearest to is `which = 'LM'`.
If the desired shift has a nonzero imaginary part, then `dndrv5`
or `dndrv6` should be modified to solve the problem. For eigenvalue
problems where the additional storage needed is not prohibitive,
the complex shift-invert driver `zndrv2` should be used.

*Chao Yang*

*11/7/1997*