Number of fixed n-celled self-avoiding polygons on square lattice.

Terms

a(0) =1a(1) =2a(2) =6a(3) =19a(4) =63a(5) =216a(6) =756a(7) =2684a(8) =9638a(9) =34930a(10) =127560a(11) =468837a(12) =1732702a(13) =6434322a(14) =23993874a(15) =89805691a(16) =337237337a(17) =1270123530a(18) =4796310672a(19) =18155586993a(20) =68874803609a(21) =261803388854a(22) =996971935098