a(0) = 3, a(1) = 5, a(2) = 1, and a(n) = (2^(1 + n) - 11*(-1)^n)/3 for n > 2.

Terms

a(0) =3a(1) =5a(2) =1a(3) =9a(4) =7a(5) =25a(6) =39a(7) =89a(8) =167a(9) =345a(10) =679a(11) =1369a(12) =2727a(13) =5465a(14) =10919a(15) =21849a(16) =43687a(17) =87385a(18) =174759a(19) =349529a(20) =699047a(21) =1398105a(22) =2796199a(23) =5592409a(24) =11184807a(25) =22369625a(26) =44739239a(27) =89478489a(28) =178956967a(29) =357913945