Sets with a congruence property.

Terms

a(0) =0a(1) =2a(2) =6a(3) =18a(4) =62a(5) =210a(6) =728a(7) =2570a(8) =9198a(9) =33288a(10) =121574a(11) =447394a(12) =1657008a(13) =6170930a(14) =23091222a(15) =86767016a(16) =327235610a(17) =1238188770a(18) =4698767640