a(n) = Sum_{i=0..n} (n!/(n-i)!)^2.
A006040
a(n) = Sum_{i=0..n} (n!/(n-i)!)^2.
Terms
- a(0) =1a(1) =2a(2) =9a(3) =82a(4) =1313a(5) =32826a(6) =1181737a(7) =57905114a(8) =3705927297a(9) =300180111058
External references
- oeis: A006040
A006040
a(n) = Sum_{i=0..n} (n!/(n-i)!)^2.