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The top level subroutine XYaupd  provides the reverse communication interface to IRAM or IRLM. The user directly calls subroutine XYaupd in one of the reverse communication modes to compute the eigenvalues of the linear operator defined by required for the computational mode selected by the user. Every time an operation involving and/or is needed, XYaupd prompts the user to provide the action of and/or on a vector and then re-enter XYaupd. This is the reverse communication interface.

During the initial call to subroutine XYaupd, error checking is performed on many of the input arguments. The workspace provided to XYaupd is also partitioned and various counters and pointers are initialized.

Chao Yang