Sets with a congruence property.

Terms

a(0) =0a(1) =0a(2) =4a(3) =16a(4) =60a(5) =208a(6) =726a(7) =2568a(8) =9196a(9) =33286a(10) =121572a(11) =447392a(12) =1657006a(13) =6170928a(14) =23091220a(15) =86767014a(16) =327235608a(17) =1238188768a(18) =4698767638