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

Terms

a(0) =1a(1) =2a(2) =5a(3) =14a(4) =44a(5) =151a(6) =560a(7) =2221a(8) =9353a(9) =41575a(10) =194148a(11) =948716a(12) =4834965a(13) =25624951a(14) =140886544a(15) =801808675a(16) =4714489141a(17) =28590416466a(18) =178551890345