a(n) = Sum_{k=0..n} C(n,3k)^2.

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =17a(5) =101a(6) =402a(7) =1275a(8) =3921a(9) =14114a(10) =58601a(11) =243695a(12) =950578a(13) =3537847a(14) =13166791a(15) =50514102a(16) =198627921a(17) =782913717a(18) =3054480306a(19) =11824753551a(20) =45823049817a(21) =178682390994a(22) =700285942731