Hierarchical renormalization-group study on the planar bond-percolation   problem
  For certain hierarchical structures, one can study the percolation problem using the renormalization-group method in a very precise way. We show that the idea can be also applied to two-dimensional planar lattices by regarding them as hierarchical structures. Either a lower bound or an exact critical probability can be obtained with this method and the correlation-length critical exponent is approximately estimated as $\nu \approx 1$. 
Seung Ki Baek and Petter Minnhagen
cond-mat.stat-mech