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numbere in a triangular table

Source: Czech and Slovak Olympiad 1985, National Round, Problem 5

September 11, 2024
combinatorics

Problem Statement

A triangular table with nn rows and nn columns is given, the ii-th row ends with a field in the vv-th column. In each field of the table, some of the numbers 1,2,...,n1,2,..., n are written such that for each k∈1,2,...,nk \in {1, 2,..., n} all the numbers 1,2,...,n1,2,..., n occur in the union of the kk-th row and the kk-th column. Prove that for odd nn, each of the numbers 1,2,...,n1,2,..., n is written in the last box of a row. https://cdn.artofproblemsolving.com/attachments/f/9/2aed55628edb1505c7de27c152127b04d8d991.png