a(n)=2*5^n-2^n.

Terms

a(0) =1a(1) =8a(2) =46a(3) =242a(4) =1234a(5) =6218a(6) =31186a(7) =156122a(8) =780994a(9) =3905738a(10) =19530226a(11) =97654202a(12) =488277154a(13) =2441398058a(14) =12207014866a(15) =61035123482a(16) =305175715714