Catafusenes (see references for precise definition).

Terms

a(0) =0a(1) =0a(2) =1a(3) =9a(4) =57a(5) =315a(6) =1629a(7) =8127a(8) =39718a(9) =191754a(10) =919035a(11) =4385799a(12) =20879100a(13) =99276840a(14) =471848195a(15) =2242864575a(16) =10665998760a(17) =50757180840a(18) =241743946635