A bisection of A000957.

Terms

a(0) =0a(1) =0a(2) =2a(3) =18a(4) =186a(5) =2120a(6) =25724a(7) =325878a(8) =4260282a(9) =57048048a(10) =778483932a(11) =10786724388a(12) =151355847012