(4^n+2^n+0^n+(-2)^n)/4.

Terms

a(0) =1a(1) =1a(2) =6a(3) =16a(4) =72a(5) =256a(6) =1056a(7) =4096a(8) =16512a(9) =65536a(10) =262656a(11) =1048576a(12) =4196352a(13) =16777216a(14) =67117056a(15) =268435456a(16) =1073774592a(17) =4294967296a(18) =17180000256a(19) =68719476736a(20) =274878431232