a(n) = (1/2) * Sum_{k=0..floor(n/3)} 3^(n-2*k) * binomial(2*n-4*k+2,2*k+1).

A387604

a(n) = (1/2) * Sum_{k=0..floor(n/3)} 3^(n-2*k) * binomial(2*n-4*k+2,2*k+1).

Terms

    a(0) =1a(1) =6a(2) =27a(3) =114a(4) =495a(5) =2214a(6) =9990a(7) =44982a(8) =201933a(9) =905526a(10) =4061016a(11) =18217710a(12) =81735156a(13) =366712272a(14) =1645244379a(15) =7381235808a(16) =33115172733a(17) =148568241906a(18) =666539094105

External references