Something that doesn't divide a factorial

Find the second largest two-digit number n n such that n n does not divide ( n 1 ) ! (n-1)!


Inspiration .This problem is original


The answer is 89.

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2 solutions

Nihar Mahajan
Sep 30, 2015

I claim that n n is a prime number. So , suppose n n is not a prime number. This means we can express n = p × q n=p\times q for some positive integers p , q p,q such that 1 < p , q < n 1<p,q<n . Let S S be a set such that S = { 1 , 2 , 3 , 4 , p , , q , , ( n 1 ) } S=\{1,2,3,4\dots, p , \dots , q ,\dots ,(n-1)\} . Note that ( n 1 ) ! (n-1)! is the product of all elements of S S .Thus, p ( n 1 ) ! p|(n-1)! , q ( n 1 ) ! q|(n-1)! which means p q ( n 1 ) ! n ( n 1 ) ! pq|(n-1)! \Rightarrow n|(n-1)! . So n n must be prime so as to not divide ( n 1 ) ! (n-1)! , thus the second largest two digit prime number is 89 \boxed{89} .

As pointed by Svatejas S , the answer is trivial by Wilson's Theorem.

You can solve it in just one line using Wilson's theorem.

A Former Brilliant Member - 5 years, 8 months ago

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Lol , I had forgot that theorem. Yes one liner by it.

Nihar Mahajan - 5 years, 8 months ago

May you explain me please this wilson theorem

Mr Yovan - 5 years, 8 months ago

Your proof is incomplete. What if p and q are the same? For instance, 4 is not prime yet it does not divide 3!. Can you prove that 4 is the only such composite for which this is true? Or might there be another one between 90 and 99?

Jason Short - 5 years, 6 months ago

This is a beautiful way of asking for the penultimate prime number between 1 and 100

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