a(n) = 2^(n + 1) * (2^n + 1) - 1.

Terms

a(0) =3a(1) =11a(2) =39a(3) =143a(4) =543a(5) =2111a(6) =8319a(7) =33023a(8) =131583a(9) =525311a(10) =2099199a(11) =8392703a(12) =33562623a(13) =134234111a(14) =536903679a(15) =2147549183a(16) =8590065663a(17) =34360000511a(18) =137439477759a(19) =549756862463