Factorial problem by Ashu Dablo

Find the largest positive integer n, such that n ! n! can be expressed as the product of ( n 2 11 ) (n-2^{11}) consecutive integers.

Let a positive integer x x be n ( m o d 71 × 73 × 79 ) \equiv n \pmod{71\times 73\times 79}

Find x x ( m o d 41 × 47 ) \pmod{41\times 47}

x is the remainder when n is divided by 409457 (409457=71x73x79).

The answer is the remainder when x is divided by 1927. (1927=41*47)

This problem is inspired from Interesting property of 7! by Agnishom Chattopadhyay.


The answer is 932.

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1 solution

Trevor Arashiro
Dec 9, 2014

Lemma: the largest integer n that can be represented by ( n k ) ! (n-k)! consecutive integers is n = ( k + 1 ) ! 1 n=(k+1)!-1 .

I don't know a good way to explain this (or just the fact that I'm horrible at explaining things) but here it goes.

Imagine the first k numbers of ( n ) ! (n)! they are 2 3 4.... k 2\cdot3\cdot4....k

Now, We want to add another number to this while still minimizing k (explained later)

The first k+1 numbers become 1 2 3 4... k 1\cdot 2\cdot 3 \cdot 4...k

Now, think of n! And ( n + 1 ) ! (n+1)! all their numbers overlap except for n+1. So ( n + 1 ) ! (n+1)! has one more number than n!

Now, remove the first k+1 digits (since we're adding one more number (this number is n+1)) and we get n ! = ( n + 1 ) n ! ( k + 1 ) ! n!=\dfrac{(n+1)n!}{(k+1)!} . (This is why we want to minimize k+1)

This implies ( k + 1 ) ! 1 = n (k+1)!-1=n


Now, for the rest of our solution

Here, n = ( 2 11 + 1 ) ! 1 n=(2^{11}+1)!-1

It's obvious that 2 11 2^{11} is greater than 79. Thus ( 2 11 + 1 ) (2^{11}+1) contains 79,73,71 in it prime factors and is thus is 0 ( m o d 79 , 73 , 71 ) \equiv 0 \pmod{79,73,71}

Since we are subtracting one, it will be 1 ( m o d 409457 ) \equiv -1 \pmod {409457}

Or 409456 ( m o d 409457 ) \equiv 409456 \pmod{409457}

By some calculations 409456 932 ( m o d 1927 ) 409456\equiv \boxed{932 \pmod{1927}}

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