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) =819a(7) =3382a(8) =14067a(9) =58927a(10) =248303a(11) =1051128a(12) =4466787a(13) =19043766a(14) =81418746a(15) =348936288a(16) =1498601459a(17) =6448162221a(18) =27791057997a(19) =119954739879a(20) =518451715551