Number of n-celled polygons with perimeter 2n+2 on square lattice.

Terms

a(0) =1a(1) =2a(2) =6a(3) =18a(4) =55a(5) =174a(6) =566a(7) =1868a(8) =6237a(9) =21050a(10) =71666a(11) =245696a(12) =847317a(13) =2937116a(14) =10226574a(15) =35746292a(16) =125380257a(17) =441125966a(18) =1556301578a(19) =5504340656a(20) =19511769838a(21) =69307575896a(22) =246650185778a(23) =879290186630