The Quantum O(N) Heisenberg Antiferromagnet for N -> infinityy
  We study the O(N) quantum Heisenberg antiferromagnet using a parametrisation in terms of real fermions. The N->infinity limit of the model is controlled by a saddle point that is infinitely degenerate in many cases and which represents singlet states on dimers that cover the lattice. Our results are similar to results reported previously on an SU(N) generalisation of the quantum Heisenberg antiferromagnet for N->infinity. 
D. Foerster, F. Triozon