a(n) = (3*10^n + 2^n)/4.

Terms

a(0) =1a(1) =8a(2) =76a(3) =752a(4) =7504a(5) =75008a(6) =750016a(7) =7500032a(8) =75000064a(9) =750000128a(10) =7500000256a(11) =75000000512a(12) =750000001024