%N Smallest size (greater than 1) of a topology that cannot be made using n points.
%D K. Ragnarsson and B. E. Tenner, Obtainable sizes of topologies on finite sets, J. Combin. Theory Ser. A 117 (2010) 138-151.
%e There is no topology on 3 points having 7 open sets. However, there do exist topologies on 3 points that have 2, 3, 4, 5 and 6 open sets.
%Y Cf. A137813.
%A _Bridget Tenner_, Feb 11 2008