Number of free polyominoes (or square animals) with n cells.

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =5a(5) =12a(6) =35a(7) =108a(8) =369a(9) =1285a(10) =4655a(11) =17073a(12) =63600a(13) =238591a(14) =901971a(15) =3426576a(16) =13079255a(17) =50107909a(18) =192622052a(19) =742624232a(20) =2870671950a(21) =11123060678a(22) =43191857688a(23) =168047007728a(24) =654999700403