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Integers with determinant \pm 1

Source: INMO 2021 Problem 1

March 7, 2021
determinantIntegersnumber theoryINMO

Problem Statement

Suppose r2r \ge 2 is an integer, and let m1,n1,m2,n2,,mr,nrm_1, n_1, m_2, n_2, \dots, m_r, n_r be 2r2r integers such that minjmjni=1\left|m_in_j-m_jn_i\right|=1 for any two integers ii and jj satisfying 1i<jr1 \le i<j \le r. Determine the maximum possible value of rr.
Proposed by B Sury