Fourgonal extensions (Q1380851)

The paper introduces fourgonal extensions as special cases of fourgonal families. Fourgonal families provide an algebraic description of a certain class of generalized quadrangles. The author proves that two fourgonal extensions describe the same generalised quadrangle if and only if the fourgonal extensions are isomorphic. The generalized quadrangles obtained from fourgonal extensions are precisely those elation generalized quadrangles whose elation group has a central subgroup with specific properties.