(k-1)/2 where k runs over odd terms of A001372.

Terms

a(0) =0a(1) =0a(2) =1a(3) =3a(4) =9a(5) =23a(6) =171a(7) =475a(8) =1307a(9) =10245a(10) =28951a(11) =233099a(12) =664496a(13) =5442024a(14) =3109434694a(15) =218785948496