8^n+2^n-1^n.

Terms

a(0) =1a(1) =9a(2) =67a(3) =519a(4) =4111a(5) =32799a(6) =262207a(7) =2097279a(8) =16777471a(9) =134218239a(10) =1073742847a(11) =8589936639a(12) =68719480831a(13) =549755822079