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largest possible total length of the walls, on a mza of n^2 unit square cells

Source: ITAMO 1987 p7

February 2, 2020
combinatorics

Problem Statement

A square paper of side nn is divided into n2n^2 unit square cells. A maze is drawn on the paper with unit walls between some cells in such a way that one can reach every cell from every other cell not crossing any wall. Find, in terms of nn, the largest possible total length of the walls.