Determine the smallest integer m > 8 , such that the equation
⌊ n + 2 n 3 + m ⌋ = n + 2 n 3 + m
has at least eleven odd positive solutions and eleven even positive solutions for n ?
Notation:
⌊
⋅
⌋
denotes the
floor function
.
This problem is a part of <Christmas Streak 2017> series .
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Note: The claim of "If m is minimal, then the number of odd divisors and even divisors should be equal" is not necessarily true.
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Think about it, if there are even divisors, then the number must contain 2 .
However, if the number is 2 n ⋅ k , where n ≥ 2 and k is odd, then the number of odd divisors is the same with 2 k .
But the number of even divisors are way more than 2 k .
And since the problem asked for at least 1 1 odd divisors and 1 1 even divisors, we can infer that the number contains only one 2 , and therefore the number has equal amount of odd and even divisors.
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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 with (say) 11 odd and 25 even divisors. In that case, the minimal m will not have an equal number of odd and even divisors.
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⌊ n + 2 n 3 + m ⌋ = n + 2 n 3 + m means that n + 2 n 3 + m is a natural number.
Therefore n + 2 ∣ n 3 + m .
Knowing that n 3 + 8 = ( n + 2 ) ( n 2 − 2 n + 4 ) , we can figure that n + 2 ∣ m − 8 .
Then since m + 2 > 2 , m − 8 must have at least 24 distinct divisors, including 1 and 2 .
Since there are even divisors, the number must contain 2 .
However, if the number is 2 n ⋅ k where n ≥ 2 and k is odd, then the number of odd divisors is the same with 2 k .
But the number of even divisors are way more than 2 k .
And since the problem asked for at least 1 1 odd divisors and 1 1 even divisors, we can infer that the number contains only one 2 and therefore the number has equal amount of odd and even divisors.
So we can infer that m − 8 = 2 k where k is an odd number.
k is a minimal number with 1 2 distinct divisors. Therefore, k = 3 2 ⋅ 5 ⋅ 7 .
∴ m = 2 ⋅ 3 2 ⋅ 5 ⋅ 7 + 8 = 6 3 8 .