README.md

# 1D XY spin-half model

The longitudinal (SzSz) correlation is the same than the
density-density correlation in a free fermionic chain (up to a shift
in the reciprocal space)

The transverse (SxSx) correlation is however computed using a pfaffian
operation over an antisymetric matrix due to Wick theorem

All results can be found in Cruz and Goncalvez publication (different
sign due to fermion and not hole mapping)