@article{white1992,
  author = {White, S. R.},
  title = {{Density Matrix Formulation for Quantum Renormalization Groups}},
  journal = {Physical Review Letters},
  volume = {69},
  number = {19},
  pages = {2863--2866},
  year = {1992},
  doi = {10.1103/PhysRevLett.69.2863}
}