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

Terms

a(0) =0a(1) =4a(2) =20a(3) =102a(4) =520a(5) =2570a(6) =12300a(7) =57358a(8) =262160a(9) =1179666a(10) =5242900a(11) =23068694a(12) =100663320a(13) =436207642a(14) =1879048220a(15) =8053063710a(16) =34359738400a(17) =146028888098a(18) =618475290660