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

Terms

a(0) =0a(1) =0a(2) =1a(3) =1a(4) =7a(5) =10a(6) =40a(7) =70a(8) =217a(9) =427a(10) =1159a(11) =2440a(12) =6160a(13) =13480a(14) =32689a(15) =73129a(16) =173383a(17) =392770a(18) =919480a(19) =2097790a(20) =4875913a(21) =11169283a(22) =25856071a(23) =59363920a(24) =137109280a(25) =315201040a(26) =727060321a(27) =1672663441a(28) =3855438727a(29) =8873429050