Divisible by this year? (Part 3: What if I did this?)

n ! n! or n n -factorial is the product of all integers from 1 1 up to n n ( n ! = 1 × 2 × 3 × . . . × n ) (n! = 1 \times 2 \times 3 \times ... \times n) . Let's denote n ! ! n!! be the product of all factorials from 1 ! 1! up to n ! n! ( n ! ! = 1 ! × 2 ! × 3 ! × . . . × n ! ) (n!! = 1! \times 2! \times 3! \times ... \times n!) . Find the maximum integral value of k k such that 201 4 k 2014^k divides 2014 ! ! 2014!!

You may also try these problem:

Divisible by this year???

Divisible by this year??? (Part 2: Factorials)

This problem is part of the set " Symphony "


The answer is 37297.

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1 solution

Jake Lai
Dec 14, 2014

In this question, we only need to consider the prime factors of 2014: 2, 19, and 53.

Since 2, 19, and 53 are pairwise coprime , we will not overcount and thus we can further simplify the question by looking at the maximal k k for which 5 3 k 2014 ! ! 53^k | 2014!! .

Notice that 2014 ! ! = 1 2014 × 2 2013 × × 2014 2014!! = 1^{2014} \times 2^{2013} \times \ldots \times 2014 .

We begin by checking all factors of 2014 ! ! 2014!! of the form ( 53 n ) 2015 53 n (53n)^{2015-53n} ; they have 5 3 2015 53 n 53^{2015-53n} as a factor, and there are 38 such integers; hence, a lower bound for k = n = 1 38 2015 53 n = 37297 k = \displaystyle \sum_{n=1}^{38} 2015-53n = 37297 .

Then, we check all factors of the form ( 5 3 2 n ) 2015 5 3 2 n (53^{2}n)^{2015-53^{2}n} ; however, because 5 3 2 > 2014 ! 53^{2} > 2014! , there are no more factors to check and hence our answer is k = 37297 k = \boxed{37297} .

Hahha thanks for posting the solution! I'm too lazy to post a lengthy one :)

Rindell Mabunga - 6 years, 6 months ago

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Nah, not a problem.

Jake Lai - 6 years, 6 months ago

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