Skip over navigation
Consider numbers of the form $u_n = 1! + 2! + 3! +...+n!$.
How many such numbers are perfect squares?
Squaresearch
Age 14 to 16
Challenge Level 
Consider numbers of the form $u_n = 1! + 2! + 3! +...+n!$.
How many such numbers are perfect squares?
You may also like
Factorial
How many zeros are there at the end of the number which is the product of first hundred positive integers?
Fac-finding
Lyndon chose this as one of his favourite problems. It is accessible but needs some careful analysis of what is included and what is not. A systematic approach is really helpful.