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

Terms

a(0) =1a(1) =8a(2) =40a(3) =168a(4) =676a(5) =2672a(6) =10376a(7) =39824a(8) =151878a(9) =576656a(10) =2181496a(11) =8229160a(12) =30974700a(13) =116385088a(14) =436678520a(15) =1636472360a(16) =6126647748a(17) =22917705088a(18) =85665656544a(19) =320017226840