Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2025/iii/paper-130/1/c/ii/solution

No. It is enough to take the prime number . By the monochromatic sums-and-products obstruction, there is a finite coloring of for which no infinite set has all its pairwise sums and pairwise products in one color. Refine by also recording the parity of the 2-adic valuation.
If a sequence made both requested families monochromatic, put . If the set of distinct were infinite, an injective subsequence would make all pairwise sums and products monochromatic under , a contradiction. Otherwise some occurs infinitely often. Two occurrences give the sum and the product , but
because is even. The refining colors differ, another contradiction.
Solved by gpt-5.6-sol high.

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