a(n) = 2^(3*n+1) - 2*n*(2*n+1).

Terms

a(0) =2a(1) =10a(2) =108a(3) =982a(4) =8120a(5) =65426a(6) =524132a(7) =4194094a(8) =33554160a(9) =268435114a(10) =2147483228a(11) =17179868678a(12) =137438952872