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

Terms

a(0) =0a(1) =0a(2) =1a(3) =6a(4) =27a(5) =104a(6) =369a(7) =1242a(8) =4039a(9) =12828a(10) =40077a(11) =123758a(12) =379011a(13) =1153872a(14) =3498025a(15) =10572354a(16) =31884543a(17) =96010436a(18) =288788613a(19) =867967830a(20) =2607282235a(21) =7828953720a(22) =23501774241a(23) =70536546986a(24) =211674885687a(25) =635160738924