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

Terms

a(0) =1a(1) =0a(2) =4a(3) =4a(4) =12a(5) =40a(6) =44a(7) =224a(8) =304a(9) =992a(10) =2208a(11) =4480a(12) =13200a(13) =24320a(14) =68608a(15) =145856a(16) =345920a(17) =848256a(18) =1834432a(19) =4644864a(20) =10239488a(21) =24708096a(22) =57602048a(23) =132493312a(24) =318103808a(25) =724885504a(26) =1728687104a(27) =4003968000a(28) =9371413504a(29) =22045935616