A bisection of A000957.

Terms

a(0) =1a(1) =1a(2) =6a(3) =57a(4) =622a(5) =7338a(6) =91144a(7) =1174281a(8) =15548694a(9) =210295326a(10) =2892818244a(11) =40347919626a(12) =569274150156