a(n) = 3*2^n + 3^n + 6.

Terms

a(0) =10a(1) =15a(2) =27a(3) =57a(4) =135a(5) =345a(6) =927a(7) =2577a(8) =7335a(9) =21225a(10) =62127a(11) =183297a(12) =543735a(13) =1618905a(14) =4832127a(15) =14447217a(16) =43243335a(17) =129533385a(18) =388206927a(19) =1163834337a(20) =3489930135a(21) =10466644665a(22) =31393642527a(23) =94168344657a(24) =282479868135a(25) =847389272745