a(1) = 1, a(n) = Sum_{k=1..n} a(n-1) + k.

Terms

a(0) =1a(1) =5a(2) =21a(3) =94a(4) =485a(5) =2931a(6) =20545a(7) =164396a(8) =1479609a(9) =14796145a(10) =162757661a(11) =1953092010a(12) =25390196221a(13) =355462747199