a(n) = a(1)*a(n-1) + a(2)*a(n-2) + ... + a(n-1)*a(1) for n >= 4.

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =8a(5) =23a(6) =68a(7) =207a(8) =644a(9) =2040a(10) =6558a(11) =21343a(12) =70186a(13) =232864a(14) =778550a(15) =2620459a(16) =8872074a(17) =30195288a(18) =103246502a(19) =354508628a(20) =1221846856a(21) =4225644866a(22) =14659644348a(23) =51002664023a(24) =177909901566a(25) =622093882290