a(n) = (11^n + 7^n)/2.

Terms

a(0) =1a(1) =9a(2) =85a(3) =837a(4) =8521a(5) =88929a(6) =944605a(7) =10155357a(8) =110061841a(9) =1199150649a(10) =13109949925a(11) =143644498677