a(n) = Sum_{i=0..n-1} a(i)*a(n-i), with a(0) = 1 and a(1) = 4.

Terms

a(0) =1a(1) =4a(2) =4a(3) =20a(4) =52a(5) =228a(6) =804a(7) =3444a(8) =13780a(9) =59588a(10) =253252a(11) =1113556a(12) =4892276a(13) =21860260a(14) =98055780a(15) =444148020a(16) =2021194260a(17) =9257373060a(18) =42583930500a(19) =196811777940a(20) =913015265460