A strange equation?

Number Theory Level pending

How many solutions are there to the equation n ! = k n 3 n!=kn^{3} where 1 n 50 1 \leq n \leq 50 and k k is a positive integer?


Notation: ! ! is the factorial notation. For example, 8 ! = 1 × 2 × 3 × × 8 8! = 1\times2\times3\times\cdots\times8 .


The answer is 24.

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1 solution

Kushal Bose
Jan 23, 2017

n ! = k n 3 = > ( n 1 ) ! = k n 2 n!=k n^3 \\ => (n-1)!=k n^2 .

It is obvious that no prime will satisfy the above equation because ( n 1 ) ! (n-1)! does not contain the prime n n .

So, at least n n will be a composite number.Let n = p 1 k 1 p 2 k 2 . . . . . p r k r n 2 = p 1 2 k 1 p 2 2 k 2 . . . . . p r 2 k r n=p_1^{k_1} p_2^{k_2}.....p_r^{k_r} \implies n^2=p_1^{2 k_1} p_2^{2 k_2}.....p_r^{2 k_r} .So, in the ( n 1 ) ! (n-1)! all these factors should be present.Let us assume that m a x { p 1 , p 2 , p 3 , , , , , p r } = p r max \{p_1,p_2,p_3,,,,,p_r\}=p_r ..To satisfy the equation we must have

2 k r p r ( n 1 ) \large 2 k_{r} p_r \leq (n-1)

With the help of these condition we can easily find solutions in the given range.

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