Similar to Bell numbers (A000110).

Terms

a(0) =1a(1) =2a(2) =6a(3) =22a(4) =92a(5) =426a(6) =2150a(7) =11708a(8) =68282a(9) =423948a(10) =2788230a(11) =19341952a(12) =141003552a(13) =1076787624a(14) =8589843716a(15) =71404154928a(16) =617151121998