a(n) = a(n-1) + 2*a(n-2) + 2, for n>=3, where a(0)= 1, a(1)= 2, a(2)= 4.

Terms

a(0) =1a(1) =2a(2) =4a(3) =10a(4) =20a(5) =42a(6) =84a(7) =170a(8) =340a(9) =682a(10) =1364a(11) =2730a(12) =5460a(13) =10922a(14) =21844a(15) =43690a(16) =87380a(17) =174762a(18) =349524a(19) =699050a(20) =1398100a(21) =2796202a(22) =5592404a(23) =11184810a(24) =22369620a(25) =44739242a(26) =89478484a(27) =178956970a(28) =357913940a(29) =715827882