a(n) = Sum{k=0..n} C(n,3k+1)^2.

Terms

a(0) =0a(1) =1a(2) =4a(3) =9a(4) =17a(5) =50a(6) =261a(7) =1275a(8) =5028a(9) =17253a(10) =58601a(11) =218042a(12) =876789a(13) =3537847a(14) =13783018a(15) =52301709a(16) =198627921a(17) =767778786a(18) =3010327497a(19) =11824753551a(20) =46200429186a(21) =179787741723a(22) =700285942731