Spin decimation
= Spin decimation
<Spin decimation> sums over spins at selected lattice sites while retaining the others. For alternate-site elimination in a nearest-neighbour periodic chain of even length, the coarse <spin-chain transfer matrix> is $W^2$, because its $(s,t)$ entry sums $W_{sa}W_{at}$ over the eliminated middle spin. Exact preservation $\operatorname{tr}(W^2)^{N/2}=\operatorname{tr}W^N$ requires keeping the scalar normalization as well as the entry ratios.