Splitting lemma (functions)

In mathematics, especially in singularity theory, the splitting lemma is a useful result due to René Thom which provides a way of simplifying the local expression of a function usually applied in a neighbourhood of a degenerate critical point.

Formal statement

Let $f:({ℝ}^n,0)\to (ℝ,0)$ be a smooth function germ, with a critical point at 0 (so $(\partial f/\partial x_i)(0)=0$ for $i=1,\dots ,n$). Let V be a subspace of ${ℝ}^n$ such that the restriction f |_V is non-degenerate, and write B for the Hessian matrix of this restriction. Let W be any complementary subspace to V. Then there is a change of coordinates $\mathrm{\Phi }(x,y)$ of the form $\mathrm{\Phi }(x,y)=(\varphi (x,y),y)$ with $x\in V,y\in W$, and a smooth function h on W such that

$f\circ \mathrm{\Phi }(x,y)=\frac{1}{2}{x}^{T}Bx+h(y).$

This result is often referred to as the parametrized Morse lemma, which can be seen by viewing y as the parameter. It is the gradient version of the implicit function theorem.

Extensions

There are extensions to infinite dimensions, to complex analytic functions, to functions invariant under the action of a compact group, ...

References

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