* a seriously low level implementation of a critical part of an algorithm is required.

*/

namespacearpackWrapper{

///@brief: Solves Ax = lambda*x for x, this calls the Arpack Routine dsaupd

/**

* @brief: Solves Ax = lambda*x for x, this calls the Arpack Routine dsaupd, executes the Lanczos method

* @param _x on input can be start value for iterative scheme on output it is the eigenvector

* @param _A operator data as double array

* @param _ev on output contains the requested eigenvalue

* @param _k number of eigenvalues wanted

* @param _n dimension of the operator

* @param _resid work array for the algorithm,

* @param _maxiter number of maximal iterations, when reached will exit with error!

* @param _eps accuracy of the eigenvalue, highly influeces the number of iterations needed

* @param _ritzoption specifies if the smallest or largest eigenvalue is needed, values can be arpack::which::smallest_algebraic and arpack::which::largest_algebraic

* @param _info value which specifies if _x should be used as start vector or if the algorithm should be initialized randomly by the routine

///@brief: Solves Ax = lambda*x for x, this calls the Arpack Routine dsaupd

/**

* @brief: Solves Ax = lambda*x for x, this calls the Arpack Routine dsaupd, executes the Lanczos method

* @param _x on input can be start value for iterative scheme on output it is the eigenvector

* @param _op operator data as a Tensor network

* @param _ev on output contains the requested eigenvalue

* @param _k number of eigenvalues wanted

* @param _n dimension of the operator

* @param _resid work array for the algorithm,

* @param _maxiter number of maximal iterations, when reached will exit with error!

* @param _eps accuracy of the eigenvalue, highly influeces the number of iterations needed

* @param _ritzoption specifies if the smallest or largest eigenvalue is needed, values can be arpack::which::smallest_algebraic and arpack::which::largest_algebraic

* @param _info value which specifies if _x should be used as start vector or if the algorithm should be initialized randomly by the routine