Coefficient of x^n in the expansion of ( 1/(1-x) * (1+x^3) )^n.

Terms

a(0) =1a(1) =1a(2) =3a(3) =13a(4) =51a(5) =201a(6) =813a(7) =3333a(8) =13779a(9) =57361a(10) =240153a(11) =1010109a(12) =4264989a(13) =18066777a(14) =76745763a(15) =326796213a(16) =1394494803a(17) =5961639969a(18) =25528971369a(19) =109482236013a(20) =470145451401