# correlation.m

Correlation order selection function - keeps only the specified orders of spin correlation in the state vector. This is useful as a replacement for gradients and phase cycles because correlation order filtering can be accomplished analytically, by just picking out the required correlation orders and zeroing everything else.

## Syntax

    rho=correlation(spin_system,rho,correlation_orders,spins)


## Arguments

  rho     -  a state vector or a horizontal stack thereof

orders  -  a row vector of correlation orders to keep

spins   -  which spins to consider (e.g. '1H', '13C', 'all')


## Outputs

  rho     - the state vector with the undesired orders of
spin correlations zeroed out


## Notes

Because quantum number information is required for this function to work, it is restricted to sphten-liouv formalism. It supports Fokker-Planck direct products.