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

Terms

a(0) =1a(1) =3a(2) =9a(3) =27a(4) =83a(5) =267a(6) =909a(7) =3267a(8) =12235a(9) =46983a(10) =182529a(11) =711099a(12) =2764619a(13) =10704147a(14) =41257341a(15) =158371011a(16) =605932099a(17) =2312728095a(18) =8812918161a(19) =33549513579a(20) =127652354627a(21) =485608571547