a(n) = Sum_{k=0..floor(n/4)} 2^k * 3^(n-3*k) * binomial(2*(n-3*k),2*k).

A391757

a(n) = Sum_{k=0..floor(n/4)} 2^k * 3^(n-3*k) * binomial(2*(n-3*k),2*k).

Terms

    a(0) =1a(1) =3a(2) =9a(3) =27a(4) =87a(5) =351a(6) =1539a(7) =6723a(8) =28467a(9) =117531a(10) =479763a(11) =1955907a(12) =7998075a(13) =32807835a(14) =134797203a(15) =554013027a(16) =2276268075a(17) =9348884811a(18) =38388273219a(19) =157620118995a(20) =647198010171

External references