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

Terms

a(0) =1a(1) =6a(2) =15a(3) =36a(4) =93a(5) =258a(6) =747a(7) =2208a(8) =6585a(9) =19710a(10) =59079a(11) =177180a(12) =531477a(13) =1594362a(14) =4783011a(15) =14348952a(16) =43046769a(17) =129140214a(18) =387420543a(19) =1162261524a(20) =3486784461a(21) =10460353266a(22) =31381059675a(23) =94143178896a(24) =282429536553a(25) =847288609518