`FastExpm.fastExpm`

— Method```
fastExpm(A)
fastExpm(A; threshold=1e-6)
fastExpm(A; nonzero_tol=1e-14)
fastExpm(A; threshold=1e-6, nonzero_tol=1e-14)
```

This function efficiently implements matrix exponential for sparse and full matrices. This code is based on scaling, taylor series and squaring. Currently works only on the CPU

Two optional keyword arguments are used to speed up the computation and preserve sparsity. [1] threshold determines the threshold for the Taylor series (default 1e-6): e.g. fastExpm(A, threshold=1e-10) [2] nonzero*tol strips elements smaller than nonzero*tol at each computation step to preserve sparsity (default 1e-14) ,e.g. fastExpm(A, nonzero_tol=1e-10) The code automatically switches from sparse to full if sparsity is below 25% to maintain speed.

This code was originally developed by Ilya Kuprov (http://spindynamics.org/) and has been adapted by F. Mentink-Vigier (fmentink@magnet.fsu.edu) and Murari Soundararajan (murari@magnet.fsu.edu) If you use this code, please cite

- H. J. Hogben, M. Krzystyniak, G. T. P. Charnock, P. J. Hore and I. Kuprov, Spinach – A software library for simulation of spin dynamics in large spin systems, J. Magn. Reson., 2011, 208, 179–194.
- I. Kuprov, Diagonalization-free implementation of spin relaxation theory for large spin systems., J. Magn. Reson., 2011, 209, 31–38.