The [<b>Lefschetz Fixed Point Theorem</b>][1] is wonderful. It generalizes the Fixed Point Theorem of Brouwer, and is an indispensable tool in topological analysis of dynamical systems.

The weakest form goes like this. For any continuous function $f:X \to X$ from a triangulable space $X$ to itself, let $H_\ast f:H_\ast X\to H_\ast X$ denote the induced endomorphism of the Rational homology groups. If the alternating sum (over dimension) of the traces

$$\Lambda(f) := \sum_{d \in \mathbb{N}}(-1)^d\text{ Tr}(H_df)$$

is non-zero, then $f$ has a fixed point! When $f$ is the identity map, $\Lambda(f)$ equals the Euler characteristic of $X$.

  [1]: http://en.wikipedia.org/wiki/Lefschetz_fixed-point_theorem