a(n) = 3*a(n-1) + 3*a(n-2), n>2, a(0)=1, a(1)=2, a(2)=8.

Terms

a(0) =1a(1) =2a(2) =8a(3) =30a(4) =114a(5) =432a(6) =1638a(7) =6210a(8) =23544a(9) =89262a(10) =338418a(11) =1283040a(12) =4864374a(13) =18442242a(14) =69919848a(15) =265086270a(16) =1005018354a(17) =3810313872a(18) =14445996678a(19) =54768931650a(20) =207644784984a(21) =787241149902