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

Terms

a(0) =0a(1) =5a(2) =15a(3) =65a(4) =255a(5) =1025a(6) =4095a(7) =16385a(8) =65535a(9) =262145a(10) =1048575a(11) =4194305a(12) =16777215a(13) =67108865a(14) =268435455a(15) =1073741825a(16) =4294967295a(17) =17179869185a(18) =68719476735a(19) =274877906945