# A Morse function on a compact manifold has finitely many critical points

We still have a problem with the Morse lemma.

Let $u$ be a non-degenerate critical point of the function $f : \mathbb{M} \to \mathbb{R}.$ There are local coordinate with $u = (0, \dots, 0)$ such that $$f(x) = f(u) - x_1^2 - \dots - x_i^2 + x_{i+1}^2 + \dots + x_n^2$$ for every point $x = (x_1, \dots, x_n)$ in a small neighborhood of $u$.

A consequence of the Morse lemma is that non-degenerate critical points are isolated. In particular, a Morse function on a compact manifold has finitely many critical points.

The first part of the consequence we could understand but the second one "A Morse function on a compact manifold has finitely many critical points" we coundn't get it.

Could you please give us a hint? Thank you!

## 1 Answer

HINT:

You should prove that a closed subset consisting of isolated points of a compact space is finite. First, find an open cover of the space such that each open set of that cover intersects the set in at most one point. Now, use compactness

• "[...] one point" you said here is a critical point, right? Thank you!
– user455909
Mar 29, 2018 at 7:07
• @user557906: it's at most as large as the number of critical points, since some values at critical points can coincide May 1, 2018 at 20:36