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

Terms

a(0) =1a(1) =2a(2) =4a(3) =9a(4) =21a(5) =56a(6) =164a(7) =533a(8) =1818a(9) =6473a(10) =23546a(11) =87146a(12) =325737a(13) =1227708a(14) =4654284a(15) =17733539a(16) =67841448a(17) =260463500a(18) =1003087732a(19) =3873759682a(20) =14996820360a(21) =58188678048a(22) =226235685776a(23) =881235386179