a(n) = (2*8^n + 2^n)/3.

Terms

a(0) =1a(1) =6a(2) =44a(3) =344a(4) =2736a(5) =21856a(6) =174784a(7) =1398144a(8) =11184896a(9) =89478656a(10) =715828224a(11) =5726623744a(12) =45812985856a(13) =366503878656