Maths Olympiad Prep

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Number theory Difficulty 5.0 AIME Prove it Philippines

Problem:

How many positive perfect cubes are divisors of the product 1!2!3!10!1! \cdot 2! \cdot 3! \cdots 10!?

Solution

Solution:

We have N:=1!2!3!10!=2383175774N := 1! \cdot 2! \cdot 3! \cdots 10! = 2^{38} 3^{17} 5^{7} 7^{4}. Thus, a positive divisor of NN that is a perfect cube must be of the form 23a33b53c73d2^{3a} 3^{3b} 5^{3c} 7^{3d} for some nonnegative integers a,b,c,da, b, c, d.

We see that 3a383a \leq 38, 3b173b \leq 17, 3c73c \leq 7 and 3d43d \leq 4. Thus, there are 383=13\left\lceil \frac{38}{3} \right\rceil = 13 choices for aa, 173=6\left\lceil \frac{17}{3} \right\rceil = 6 choices for bb, 73=3\left\lceil \frac{7}{3} \right\rceil = 3 choices for cc, and 43=2\left\lceil \frac{4}{3} \right\rceil = 2 choices for dd.

Hence, there are 1363×2=46813 \cdot 6 \cdot 3 \times 2 = 468 positive perfect cube divisors of NN.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.