Christmas Streak 20/88: Both Even And Odd

Determine the smallest integer m > 8 m > 8 , such that the equation

n 3 + m n + 2 = n 3 + m n + 2 \left\lfloor\dfrac{n^3+m}{n+2}\right\rfloor=\dfrac{n^3+m}{n+2}

has at least eleven odd positive solutions and eleven even positive solutions for n n ?


Notation: \lfloor\cdot\rfloor denotes the floor function .


This problem is a part of <Christmas Streak 2017> series .


The answer is 638.

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1 solution

Boi (보이)
Oct 18, 2017

n 3 + m n + 2 = n 3 + m n + 2 \left\lfloor\dfrac{n^3+m}{n+2}\right\rfloor=\dfrac{n^3+m}{n+2} means that n 3 + m n + 2 \dfrac{n^3+m}{n+2} is a natural number.

Therefore n + 2 n 3 + m . n+2|n^3+m.

Knowing that n 3 + 8 = ( n + 2 ) ( n 2 2 n + 4 ) , n^3+8=(n+2)(n^2-2n+4), we can figure that n + 2 m 8. n+2|m-8.

Then since m + 2 > 2 , m+2>2, m 8 m-8 must have at least 24 distinct divisors, including 1 1 and 2. 2.

Since there are even divisors, the number must contain 2. 2.

However, if the number is 2 n k 2^n\cdot k where n 2 n\ge2 and k k is odd, then the number of odd divisors is the same with 2 k . 2k.

But the number of even divisors are way more than 2 k . 2k.

And since the problem asked for at least 11 11 odd divisors and 11 11 even divisors, we can infer that the number contains only one 2 2 and therefore the number has equal amount of odd and even divisors.

So we can infer that m 8 = 2 k m-8=2k where k k is an odd number.

k k is a minimal number with 12 12 distinct divisors. Therefore, k = 3 2 5 7. k=3^2\cdot5\cdot7.

m = 2 3 2 5 7 + 8 = 638. \therefore~m=2\cdot3^2\cdot5\cdot7+8=638.

Note: The claim of "If m is minimal, then the number of odd divisors and even divisors should be equal" is not necessarily true.

Calvin Lin Staff - 3 years, 7 months ago

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Think about it, if there are even divisors, then the number must contain 2. 2.

However, if the number is 2 n k , 2^n\cdot k, where n 2 n\ge2 and k k is odd, then the number of odd divisors is the same with 2 k . 2k.

But the number of even divisors are way more than 2 k . 2k.

And since the problem asked for at least 11 11 odd divisors and 11 11 even divisors, we can infer that the number contains only one 2 , 2, and therefore the number has equal amount of odd and even divisors.

Boi (보이) - 3 years, 7 months ago

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Right, that fills in the missing details of the argument.

The reason why it is "not necessarily true", is if we required the minimal m m with (say) 11 odd and 25 even divisors. In that case, the minimal m m will not have an equal number of odd and even divisors.

Calvin Lin Staff - 3 years, 7 months ago

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@Calvin Lin Oh, I see.

Thanks, I've updated the solution! ^^

Boi (보이) - 3 years, 7 months ago

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