# ``igraph`.`ARPACKOptions``

class documentation

`class ARPACKOptions:`

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Class representing the parameters of the ARPACK module.

ARPACK is a Fortran implementation of the implicitly restarted Arnoldi method, an algorithm for calculating some of the eigenvalues and eigenvectors of a given matrix. igraph uses this package occasionally, and this class can be used to fine-tune the behaviour of ARPACK in such cases.

The class has several attributes which are not documented here, since they are usually of marginal use to the ordinary user. See the source code of the original ARPACK Fortran package (especially the file `dsaupd.f`) for a detailed explanation of the parameters. Only the most basic attributes are explained here. Most of them are read only unless stated otherwise.

- `bmat`: type of the eigenproblem solved. `'I'` means standard eigenproblem (A*x = lambda*x), `'G'` means generalized eigenproblem (A*x = lambda*B*x).
- `n`: dimension of the eigenproblem
- `tol`: precision. If less than or equal to zero, the standard machine precision is used as computed by the LAPACK utility called `dlamch`. This can be modified.
- `mxiter`: maximum number of update iterations to take. This can be modified. You can also use `maxiter`.
- `iter`: actual number of update iterations taken
- `numop`: total number of OP*x operations
- `numopb`: total number of B*x operations if `bmat` is `'G'`
- `numreo`: total number of steps of re-orthogonalization