a(n) = 3^n + (-1)^n.

Terms

a(0) =2a(1) =2a(2) =10a(3) =26a(4) =82a(5) =242a(6) =730a(7) =2186a(8) =6562a(9) =19682a(10) =59050a(11) =177146a(12) =531442a(13) =1594322a(14) =4782970a(15) =14348906a(16) =43046722a(17) =129140162a(18) =387420490a(19) =1162261466a(20) =3486784402a(21) =10460353202a(22) =31381059610a(23) =94143178826a(24) =282429536482a(25) =847288609442