A246316(2^n-1).

Terms

a(0) =1a(1) =4a(2) =12a(3) =40a(4) =176a(5) =724a(6) =3276a(7) =14620a(8) =61044a(9) =250396a(10) =1012804a(11) =4076236a(12) =16344284a(13) =65471860a(14) =262071708a(15) =1048641260