a(n) = Sum_{i = 1..n} (n - i)^(1 + i).

Terms

a(0) =0a(1) =1a(2) =5a(3) =18a(4) =60a(5) =203a(6) =725a(7) =2772a(8) =11368a(9) =49853a(10) =232757a(11) =1151902a(12) =6018772a(13) =33087191a(14) =190780197a(15) =1150653904a(16) =7241710912a(17) =47454745785a(18) =323154696165