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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.
An introduction to the binomial coefficient, and exploration of some of the formulae it satisfies.
Prove that k.k! = (k+1)! - k! and sum the series 1.1! + 2.2! + 3.3! +...+n.n!
Compares the size of functions f(n) for large values of n.
The harmonic triangle is built from fractions with unit numerators using a rule very similar to Pascal's triangle.
How many zeros are there at the end of the number which is the product of first hundred positive integers?
Prove that k.k! = (k+1)! - k! and sum the series 1.1! + 2.2! + 3.3! +...+n.n!
Consider numbers of the form un = 1! + 2! + 3! +...+n!. How many such numbers are perfect squares?
How many noughts are at the end of these giant numbers?
Which is larger: (a) 1.000001^{1000000} or 2? (b) 100^{300} or 300! (i.e.factorial 300)