## The 36 officers problem

*This article is part of Five of Euler's best. Click here to read the other four problems featured in this series, which is based on a talk given by the mathematician Günter M. Ziegler at the European Congress of Mathematics 2016.*

This problem looks like a puzzle you might find in a Sunday paper. Imagine there's a war. You're in command of an army that consists of six regiments, each containing six officers of six different ranks. Can you arrange the officers in a 6x6 square so that each row and each column of the square holds only one officer from each regiment and only one officer from each rank?

Just as any Sunday puzzler Euler had a go at the problem, but without success. "After we have put a lot of thought into finding a solution, we have to admit that such an arrangement is impossible, though we can't give a rigorous demonstration of this" he wrote in 1782. A proof that the problem is impossible to solve didn't come along until nearly 130 years later. The French mathematician Gaston Terry provided it in 1901.

If you've ever come across Latin
squares, then the 36 officers problem might have reminded you of
them. A *Latin square* is a square array of symbols (for example numbers
or letters) in which every
symbol occurs just once in every row and column. If you combine two
Latin squares of the same size but with different symbols, you get what's called a
*Graeco-Latin square* (also called an Euler square). It contains pairs of symbols so that every
member of a pair occurs exactly once in each row and column, and so
that every possible pair appears exactly once on the array.

+ | = |

Euler realised that a solution of the 36
officers problem would give us a Graeco-Latin 6x6 square. The pairs in
this case represent an officer's rank and regiment. That's unlucky: if
their had been five regiments and ranks, or seven regiments and ranks,
then the problem could have been solved. Euler was aware of this too
and speculated that Graeco-Latin squares are impossible if the number
of cells on the side of square (the *order* of the square) is
of the form 4*k* + 2 for a whole number *k* (6 = 4x1
+2). It wasn't until 1960 that he was proved wrong. The mathematicians Bose,
Shrikhande and Parker enlisted the help of computers to prove that Graeco-Latin squares exist for all orders except 2 and 6.

So why do we mention what at first sight looks like an Euler fail rather than an Euler
success? Because as usual with Euler, there's a lot more to his work
on the problem than a bit of speculation. The 36 officer problem inspired
him to a lot of work on Latin and Graeco-Latin squares, including on
problems akin to modern-day sudoku (see this article
for more on sudoku and other types of interesting squares). It
contributed important work to an area of maths called
*combinatorics* which, as you may have guessed, is about
combining objects in ways that satisfy particular constraints, and
has many applications in the real world.

### About this article

This article is part of Five of Euler's best. Click here to read the other four problems featured in this series, which is based on a talk given by the mathematician Günter M. Ziegler at the European Congress of Mathematics 2016.

Günter M. Ziegler is professor of Mathematics at Freie Universität Berlin, where he also directs the Mathematics Media Office of the German Mathematical Society DMV. His books include *Proofs from THE BOOK* (with Martin Aigner) and *Do I count? Stories from Mathematics.*.

Marianne Freiberger is Editor of *Plus*. She really enjoyed Ziegler's talk about Euler at the European Congress of Mathematics in Berlin in July 2016, on which this article is based.