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

Terms

a(0) =1a(1) =0a(2) =0a(3) =2a(4) =6a(5) =0a(6) =4a(7) =72a(8) =36a(9) =8a(10) =360a(11) =1080a(12) =232a(13) =1344a(14) =10080a(15) =12128a(16) =5616a(17) =60480a(18) =181504a(19) =129312a(20) =292896a(21) =1596800a(22) =2601024a(23) =2179584a(24) =10425280a(25) =31227264a(26) =35328384a(27) =63843200a(28) =267387264a(29) =512262144