Degeneracy of the ground-state of antiferromagnetic spin-1/2 Hamiltonians G. Misguich, C. Lhuillier, M. Mambrini and P. Sindzingre Eur. Phys. J. B, 26 2 (2002) 167-183 Published online: 15 March 2002 DOI: 10.1140/epjb/e20020078