G.f.: Sum_{n>=0} x^n * Product_{k=1..n} (1 + k*x + x^2).

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =11a(5) =28a(6) =74a(7) =206a(8) =601a(9) =1826a(10) =5766a(11) =18851a(12) =63676a(13) =221678a(14) =793958a(15) =2920292a(16) =11014653a(17) =42543773a(18) =168074091a(19) =678403932a(20) =2794920078a(21) =11742254750a(22) =50266213000a(23) =219085792538a(24) =971543475593