Maths Olympiad Prep

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Problem 2279

IMO Shortlist mid-range; USAMO P2/P5
Number theory Difficulty 8.4 Find the answer IMO Shortlist

An integer nn is said to be good if n|n| is not the square of an integer. Determine all integers mm with the following property: mm can be represented, in infinitely many ways, as a sum of three distinct good integers whose product is the square of an odd integer.

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Official solution

To solve the problem, we need to determine all integers m m such that m m can be represented in infinitely many ways as a sum of three distinct good integers whose product is the square of an odd integer.

First, let's clarify the conditions:
- A number n n is said to be good if n |n| is not a perfect square. Thus, our focus is on good integers.
- The product of the three distinct good integers should be the square of an odd integer.

To explore this situation, consider three distinct integers a,b, a, b, and c c (all good), such that:
a+b+c=m a + b + c = m
and
abc=k2 abc = k^2
where k k is an odd integer.

Since abc=k2 abc = k^2 , and k k is assumed to be odd, all prime factors of abc abc must occur with an even multiplicity. Consequently, each of a,b, a, b, and c c must have an even count of each prime factor (except possibly a shared factor of 1-1 if some are negative), making them products of (not necessarily distinct) prime squares. However, all must remain good, i.e., not themselves squares.

Next, consider possible constructions and examine specific m m values:
- If each pair (a,b,c)(a, b, c) contains exactly two terms such that their product contributes odd prime squares, various combinations can be attempted:
- For example, choosing a,b, a, b, or c c as small odd integers satisfying the good condition ensures they are not perfect squares, yet their multiplication satisfies abc=k2 abc = k^2.

A broader solution requires understanding that the oddness ensures versatility in the component choices, enabling algebraic manipulation in constructing valid sets that yield infinitely many m m .

To find all m m with this property, note that only specific constructions imply infinite multiplicity:
- Generally, if m=0 m = 0 , we can consistently choose negative supplements for squares and positives appropriately to manipulate unique differences. This method is adaptable due to multilinear conditions across infinite tuples.

Thus, the integer m m that can be represented, in infinitely many ways, as a sum of three good integers with the appropriate properties is simply:
0 \boxed{0}

Given the formulation and the unique allowance for even multiplicity through prime factor interactions among odd components, m=0 m = 0 is the appropriate outcome under these constructions.

This showcases the scenario of symmetric construction, emphasizing negative pair symmetry in perfect square balance with k2, k^2, sustaining the infinite representation requirement.

Source: Omni-MATH, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.