Quantum site percolation on triangular lattice and the integer quantum   Hall effect
  Generic classical electron motion in a strong perpendicular magnetic field and random potential reduces to the bond percolation on a square lattice. Here we point out that for certain smooth 2D potentials with 120 degrees rotational symmetry this problem reduces to the site percolation on a triangular lattice. We use this observation to develop an approximate analytical description of the integer quantum Hall transition. For this purpose we devise a quantum generalization of the real-space renormalization group (RG) treatment of the site percolation on the triangular lattice. In quantum case, the RG transformation describes the evolution of the distribution of the $3\times 3$ scattering matrix at the sites. We find the fixed point of this distribution and use it to determine the critical exponent, $\nu$, for which we find the value $\nu \approx 2.3-2.76$. The RG step involves only a single Hikami box, and thus can serve as a minimal RG description of the quantum Hall transition. 
V. V. Mkhitaryan and M. E. Raikh