a(n) = Sum_{d|n} phi(d)*3^(n/d).

Terms

a(0) =0a(1) =3a(2) =12a(3) =33a(4) =96a(5) =255a(6) =780a(7) =2205a(8) =6672a(9) =19755a(10) =59340a(11) =177177a(12) =532416a(13) =1594359a(14) =4785228a(15) =14349525a(16) =43053504a(17) =129140211a(18) =387441756a(19) =1162261521a(20) =3486844320a(21) =10460357775a(22) =31381236876a(23) =94143178893a(24) =282430082832a(25) =847288610475