a(n) = Sum_{k=0..floor(n/6)} C(n-3k,3k) * 3^k * 2^(n-6k).

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =16a(5) =32a(6) =67a(7) =152a(8) =376a(9) =992a(10) =2704a(11) =7424a(12) =20233a(13) =54398a(14) =144112a(15) =376736a(16) =974368a(17) =2500544a(18) =6385435a(19) =16264220a(20) =41396788a(21) =105423776a(22) =268818064a(23) =686499008a(24) =1755723793a(25) =4495691834a(26) =11521647916a(27) =29543647160a(28) =75774096832a(29) =194353495424