2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/151650We describe some antiferromagnetic systems which exhibit spin gaps. We also discuss the effect of doping one such system, namely, the spin-ladders, with holes. Some model antiferromagnetic systems with spin gap are reviewed for which exact results are available. Exact results for a doped spin-ladder model are also mentioned.17 pages, LaTeX, 2 postscript figures, To be published in Proceedings of Conference on 70 Years of Quantum Mechanics and Recent Trends in Theoretical Physics, ed. by P. Bandyopadhyay, ISI, Calcutta, (1997)Strongly Correlated Electronstext