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

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =19a(5) =50a(6) =136a(7) =368a(8) =985a(9) =2618a(10) =6940a(11) =18392a(12) =48763a(13) =129338a(14) =343120a(15) =910304a(16) =2415025a(17) =6406898a(18) =16996852a(19) =45090728a(20) =119620579a(21) =317340098a(22) =841868632a(23) =2233386320a(24) =5924932489a(25) =15718204970a(26) =41698695820a(27) =110622122360