How Many Numbers?

Number Theory Level pending

Find the number of integers N N lying between 2 2 and 100 100 (inclusive) such that for all integers m m such that N 3 m N 2 , \dfrac{N}{3} \leq m \leq \dfrac{N}{2}, ( m N 2 m ) \dbinom{m}{N-2m} is a multiple of m . m.

Details and assumptions

  • N = 2 N=2 is considered a solution, since the only integer m m such that 2 3 m 2 2 \dfrac{2}{3} \leq m \leq \dfrac{2}{2} is 1 , 1, and ( 2 2 2 1 ) \dbinom{2}{2 - 2 \cdot 1} is a multiple of 1. 1.

  • This problem is not original.

@Calvin sir: The intended expression is ( m N 2 m ) . \dbinom{m}{N-2m}. This is valid since m N 2 m 0 m \geq N - 2m \geq 0 from the condition N 3 m N 2 . \dfrac{N}{3} \leq m \leq \dfrac{N}{2}. Roger Lu has commented why ( N N 2 m ) \dbinom{N}{N-2m} doesn't work. Please correct me if I am wrong.


The answer is 25.

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1 solution

Finn Hulse
May 23, 2014

I have a one word solution that I think will be easily understood:

PRIMES \text{PRIMES}

Any questions?

I do not understand the answer. I do not see any integer > 3 would work. For even number N=2k, (k>1), you can choose m=k and obviously it does not work. For odd number N=2k+1 you can choose m=k, and since ( N N 2 m ) \left( \begin{matrix} N \\ N-2m \end{matrix} \right) = 2k+1, which is relatively prime to m=k, that does not work either. Am I missing something?

Roger Lu - 7 years ago

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Silly typo. Apologies, fixed.

Yeah, but how do you prove it? This is literally the eighth time I'm seeing a solution which doesn't show how to approach the problem. - _ _ _ _ _ _ _ _ _ _ _ __ -

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Wait the 8th time from me or on this problem because I don't see any other solutions... :O

Finn Hulse - 7 years ago

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Eighth time on Brilliant.

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@Sreejato Bhattacharya Oh yeah. :P

Finn Hulse - 7 years ago

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