MathDB
products of 50x50 numbers on 50x50 board

Source: 2018 Argentina OMA Finals L3 p4

January 16, 2023
combinatorics

Problem Statement

There is a 50×5050\times 50 grid board.. Carlos is going to write a number in each box with the following procedure. He first chooses 100100 distinct numbers that we denote f1,f2,f3,,f50,c1,c2,c3,,c50f_1,f_2,f_3,…,f_{50},c_1,c_2,c_3,…,c_{50} among which there are exactly 5050 that they are rational. Then he writes in each box (i,j)i,j) the number ficjf_i \cdot c_j (the multiplication of fif_i by cjc_j). Determine the maximum number of rational numbers that the squares on the board can contain.