a(n) = 3^n + 7^n.

Terms

a(0) =2a(1) =10a(2) =58a(3) =370a(4) =2482a(5) =17050a(6) =118378a(7) =825730a(8) =5771362a(9) =40373290a(10) =282534298a(11) =1977503890a(12) =13841818642a(13) =96890604730a(14) =678227855818