a(n) = a(1)*a(n-1) + a(2)*a(n-2) + ...+ a(n-2)*a(2) for n >= 3.

Terms

a(0) =3a(1) =1a(2) =3a(3) =10a(4) =36a(5) =137a(6) =543a(7) =2219a(8) =9285a(9) =39587a(10) =171369a(11) =751236a(12) =3328218a(13) =14878455a(14) =67030785a(15) =304036170a(16) =1387247580a(17) =6363044315a(18) =29323149825a(19) =135700543190a(20) =630375241380