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Driver `dsdrv6` implements the Cayley spectral
transformation. Eigenvalues closest to a shift
can be obtained by computing the selected eigenvalues
of
Settings of `which` are the same as those of the shift-invert modes.
It is easy to verify that `OP` is symmetric with respect
to the (semi) inner product defined by so it is best to
use -inner products with this transformation.
The eigenvalue of the original problem and are
related by
Note that the transformation becomes ill-defined as since . The expression does have a
limit as but it would be better to use
regular inverse mode or to use shift-invert mode with when eigenvalues near the origin are sought.
The Cayley transformation has slightly different properties than the
standard shift and invert
spectral transformation used by drivers `dsdrv3` and `dsdrv4`.
To use this driver, the operations

are required.

*Chao Yang*

*11/7/1997*