a(n) = (8*4^n - 5)/3.

Terms

a(0) =1a(1) =9a(2) =41a(3) =169a(4) =681a(5) =2729a(6) =10921a(7) =43689a(8) =174761a(9) =699049a(10) =2796201a(11) =11184809a(12) =44739241a(13) =178956969a(14) =715827881a(15) =2863311529a(16) =11453246121a(17) =45812984489a(18) =183251937961a(19) =733007751849