Duplicate of A009655.

Terms

a(0) =0a(1) =0a(2) =2a(3) =6a(4) =22a(5) =100a(6) =788a(7) =8568a(8) =103416a(9) =1307808a(10) =17722512a(11) =264361680a(12) =4373968080