This was a question from this year's Simon Marais Mathematics Competition. I myself had no idea how to solve it nor did I find anyone who solved it.Any ideas please?

Three spiders try to catch a beetle in a game. They are all initially positioned on the edges of a regular dodecahedron whose edges have length $1$. At some point in time, they start moving continuously along the edges of the dodecahedron. The beetle and one of the spiders move with maximum speed 1, while the remaining two spiders move with maximum speed $\frac{1}{2018}$ .

Each player always knows their own position and the position of every other player. A player can turn around at any moment and can react to the behaviour of other players instantaneously. The spiders can communicate to decide on a strategy before and during the game. If any spider occupies the same position as the beetle at some time, then the spiders win the game.

Prove that the spiders can win the game, regardless of the initial positions of all players and regardless of how the beetle moves.

  • 1
    $\begingroup$ Oh gosh, I also participated in this year's competition. Brain was fried. $\endgroup$ Commented Oct 21, 2018 at 14:55

1 Answer 1


Solutions to the Simon Marais 2018 problems are available at the competition website. I only know of the official solution to the problem (I'd love to hear if there are any others) - the main idea is to make the fast spider mirror the beetle's movement via a reflectional symmetry of the dodecahedron, which forces the beetle to stay in one half of the dodecahedron.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .