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

Terms

a(0) =0a(1) =0a(2) =1a(3) =1a(4) =9a(5) =13a(6) =65a(7) =117a(8) =441a(9) =909a(10) =2929a(11) =6565a(12) =19305a(13) =45565a(14) =126881a(15) =309141a(16) =833049a(17) =2069613a(18) =5467345a(19) =13745797a(20) =35877321a(21) =90860509a(22) =235418369a(23) =598860405a(24) =1544728185a(25) =3940169805a(26) =10135859761a(27) =25896538981a(28) =66507086889a(29) =170093242813