@article{Drzewiński:1989,
  author  = "A. Drzewiński and J. Sznajd",
  title   = "Real space renormalization group study of the anisotropic quantum Heisenberg model on a square lattice",
  journal = "Physics Letters A",
  volume  = "Vol. 138",
  number  = "no 3",
  pages   = "143--146",
  year    = "1989",
}