 Migdal-Kadanoff approximation

A real-space approximation that moves or combines lattice bonds so that selected intermediate spins can be summed through a chain recursion. On a one-dimensional nearest-neighbour Ising model, no transverse bond moving is needed and exact zero-field Ising-chain decimation closes exactly. In higher dimensions, neglected geometric correlations generally make the transformed model approximate on the original lattice. The order of bond moving and decimation must be stated; the two operations need not commute. The bond-moving-first convention is described in Position Space Renormalization Group by Mehran Kardar lecture 14, section V.D.