Is it possible to cover a $13\times 13$ chessboard with forty-two pieces of dimensions $4\times 1$ such that only the central square of the chessboard remains uncovered?
Problem
Source: Baltic Way 1998
Tags: combinatorics proposed, combinatorics
17.01.2011 20:37
Let $(i,j)$ denote the square in both the $i$-th row and $j$th column. Then the idea is to colour $(1,1)$ and $(1,2)$ black,$(1,2) $and $(2,2)$ white, $(3,1)$ and $(3,2)$ black ....... $(13,1)$ and $(13,2)$ black, $(1,3)$ and $(1,4)$ white, $(2,3)$ and $(2,4)$ black etc for the rest of that double column and there after $(i,j)$ has the same colour as $(i,j-4)$. I'll edit my post later to add a picture. From this colouring clearly any 4x1 block will cover two white squares and two black squares. There are 85 black squares and 83 white squares (recalling that the central white square is removed). Contradiction - $2\nmid 83,85$.
12.11.2024 05:03
This is an nice problem and could be dn easily by introducing complex number here