O.g.f.: A(x) = Sum_{n>=0} x^n*Product_{k=0..n} (1 + k*x).

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =9a(5) =22a(6) =57a(7) =157a(8) =453a(9) =1368a(10) =4296a(11) =13995a(12) =47138a(13) =163779a(14) =585741a(15) =2152349a(16) =8113188a(17) =31326760a(18) =123748871a(19) =499539900a(20) =2058542819a(21) =8651755865a(22) =37054078481a(23) =161591063250a(24) =717032333816