a(n) = Sum_{k=2..n} 4^k.

Terms

a(0) =0a(1) =16a(2) =80a(3) =336a(4) =1360a(5) =5456a(6) =21840a(7) =87376a(8) =349520a(9) =1398096a(10) =5592400a(11) =22369616a(12) =89478480a(13) =357913936a(14) =1431655760a(15) =5726623056a(16) =22906492240a(17) =91625968976a(18) =366503875920