Divide a triangle *T* into lots of baby triangles, so that baby triangles only meet at a common edge or a common vertex. Label each main vertex of the whole triangle by 1, 2, or 3; then label vertices on the (12) side by either 1 or 2, on the (23) side by either 2 or 3, and the (13) side by either 1 or 3. Label the points in the interior by any of 1, 2, or 3. For instance, see Figure 1.

Fun Fact: any such labelling must contain an baby (123) triangle! (In fact, there must be an *odd* number!)

Actually, a version of Sperner's Lemma holds in all dimensions. Can you figure out how it generalizes?

Sperner's Lemma is equivalent to the Brouwer fixed point theorem.

**Presentation Suggestions:**

Have everyone make their own labelled triangle and see how many (123) triangles they have in their picture.

**The Math Behind the Fact:**

There are many proofs of this fact. Some short non-constructive proofs rely on parity arguments. Constructive proofs are the key to many fixed point algorithms as well as fair division procedures. See the reference.

**How to Cite this Page:**

Su, Francis E., et al. “Sperner's Lemma.” *Math Fun Facts*.

**References:**

F.E. Su, “Rental harmony: Sperner's lemma in fair division”,

Amer. Math. Monthly, 1999.

**Fun Fact suggested by:**

Francis Su