1.Mathematisches Seminar,Universit?t zu Kiel,Kiel,Germany;2.School of Economics, Mathematics and Statistics,Birkbeck, University of London,London,UK
Abstract:
Let G be a permutation group acting on a set Ω of size n∈ℕ and let 1≤k<(n−1)/2. Livingstone and Wagner proved that the number of orbits of G on k-subsets of Ω is less than or equal to the number of orbits on (k+1)-subsets. We investigate the cases when equality occurs.