In mathematics, the Morse–Palais lemma is a result in the calculus of variations and theory of Hilbert spaces. Roughly speaking, it states that a smooth enough function near a critical point can be expressed as a quadratic form after a suitable change of coordinates.

The Morse–Palais lemma was originally proved in the finite-dimensional case by the American mathematician Marston Morse, using the Gram–Schmidt orthogonalization process. This result plays a crucial role in Morse theory. The generalization to Hilbert spaces is due to Richard Palais and Stephen Smale.

Statement of the lemma

Let ${\displaystyle (H,\langle \cdot ,\cdot \rangle )}$ be a real Hilbert space, and let ${\displaystyle U}$ be an open neighbourhood of the origin in ${\displaystyle H.}$ Let ${\displaystyle f:U\to \mathbb {R} }$ be a ${\displaystyle (k+2)}$-times continuously differentiable function with ${\displaystyle k\geq 1;}$ that is, ${\displaystyle f\in C^{k+2}(U;\mathbb {R} ).}$ Assume that ${\displaystyle f(0)=0}$ and that ${\displaystyle 0}$ is a non-degenerate critical point of ${\displaystyle f;}$ that is, the second derivative ${\displaystyle D^{2}f(0)}$ defines an isomorphism of ${\displaystyle H}$ with its continuous dual space ${\displaystyle H^{*))$ by $${\displaystyle H\ni x\mapsto \mathrm {D} ^{2}f(0)(x,-)\in H^{*}.}$$

Then there exists a subneighbourhood ${\displaystyle V}$ of ${\displaystyle 0}$ in ${\displaystyle U,}$ a diffeomorphism ${\displaystyle \varphi :V\to V}$ that is ${\displaystyle C^{k))$ with ${\displaystyle C^{k))$ inverse, and an invertible symmetric operator ${\displaystyle A:H\to H,}$ such that $${\displaystyle f(x)=\langle A\varphi (x),\varphi (x)\rangle \quad {\text{ for all ))x\in V.}$$

Corollary

Let ${\displaystyle f:U\to \mathbb {R} }$ be ${\displaystyle f\in C^{k+2))$ such that ${\displaystyle 0}$ is a non-degenerate critical point. Then there exists a ${\displaystyle C^{k))$-with-${\displaystyle C^{k))$-inverse diffeomorphism ${\displaystyle \psi :V\to V}$ and an orthogonal decomposition $${\displaystyle H=G\oplus G^{\perp },}$$ such that, if one writes $${\displaystyle \psi (x)=y+z\quad {\mbox{ with ))y\in G,z\in G^{\perp },}$$ then $${\displaystyle f(\psi (x))=\langle y,y\rangle -\langle z,z\rangle \quad {\text{ for all ))x\in V.}$$