Consider an infinite white plane divided into square cells. For which $k$ it is possible to paint a positive finite number of cells black so that on each horizontal, vertical and diagonal line of cells there is either exactly $k$ black cells or none at all? A. Dinev, K. Garov, N Belukhov
Problem
Source: Tournament of Towns, Senior A-Level Paper, Spring 2020 , p7
Tags: Coloring, combinatorics, table