a(n) = 4^n - 4.

Terms

a(0) =-3a(1) =0a(2) =12a(3) =60a(4) =252a(5) =1020a(6) =4092a(7) =16380a(8) =65532a(9) =262140a(10) =1048572a(11) =4194300a(12) =16777212a(13) =67108860a(14) =268435452a(15) =1073741820a(16) =4294967292a(17) =17179869180a(18) =68719476732a(19) =274877906940