a(n) = 7^n + 8^n.

Terms

a(0) =2a(1) =15a(2) =113a(3) =855a(4) =6497a(5) =49575a(6) =379793a(7) =2920695a(8) =22542017a(9) =174571335a(10) =1356217073a(11) =10567261335a(12) =82560763937a(13) =646644824295