a(n) = 4^n - 3*n - 1.

Terms

a(0) =0a(1) =9a(2) =54a(3) =243a(4) =1008a(5) =4077a(6) =16362a(7) =65511a(8) =262116a(9) =1048545a(10) =4194270a(11) =16777179a(12) =67108824a(13) =268435413a(14) =1073741778a(15) =4294967247a(16) =17179869132a(17) =68719476681a(18) =274877906886