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Baltic Way 2023 Shortlist

Baltic Way 2023 number theory

Problem

For a prime number and a polynomial with integer coefficients, define be the set of integers such that there exists an integer , for which is divisible by .

Prove that there exist nonconstant polynomials and such that, for infinitely many primes, the intersection of and is empty.
Solution
We take and and prove that if then the equation has no solution. Famously, there are infinitely many primes congruent to modulo .

Recall the fact that if then the only solution to the equation is . Hence, for to hold, we need and thus which is impossible for .

Techniques

Polynomials mod pQuadratic residues