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In topology and in calculus, a round function is a scalar function $M\to {\mathbb {R} }$, over a manifold $M$, whose critical points form one or several connected components, each homeomorphic to the circle $S^{1}$, also called critical loops. They are special cases of Morse-Bott functions.

The black circle in one of this critical loops.

For instance

For example, let $M$ be the torus. Let

$K=(0,2\pi )\times (0,2\pi ).\,$

Then we know that a map

$X\colon K\to {\mathbb {R} }^{3}\,$

given by

$X(\theta ,\phi )=((2+\cos \theta )\cos \phi ,(2+\cos \theta )\sin \phi ,\sin \theta )\,$

is a parametrization for almost all of $M$. Now, via the projection $\pi _{3}\colon {\mathbb {R} }^{3}\to {\mathbb {R} }$ we get the restriction

$G=\pi _{3}|_{M}\colon M\to {\mathbb {R} },(\theta ,\phi )\mapsto \sin \theta \,$

$G=G(\theta ,\phi )=\sin \theta$ is a function whose critical sets are determined by

$\nabla G(\theta ,\phi )=\left({{\partial }G \over {\partial }\theta },{{\partial }G \over {\partial }\phi }\right)\!\left(\theta ,\phi \right)=(0,0),\,$

this is if and only if $\theta ={\pi \over 2},\ {3\pi \over 2}$.

These two values for $\theta$ give the critical sets

$X({\pi /2},\phi )=(2\cos \phi ,2\sin \phi ,1)\,$

$X({3\pi /2},\phi )=(2\cos \phi ,2\sin \phi ,-1)\,$

which represent two extremal circles over the torus $M$.

Observe that the Hessian for this function is

${\rm {Hess}}(G)={\begin{bmatrix}-\sin \theta &0\\0&0\end{bmatrix}}$

which clearly it reveals itself as of ${\rm {rankHess}}(G)=1$ at the tagged circles, making the critical point degenerate, that is, showing that the critical points are not isolated.

Round complexity

Mimicking the L-S category theory one can define the round complexity asking for whether or not exist round functions on manifolds and/or for the minimum number of critical loops.

References

• Siersma and Khimshiasvili, On minimal round functions, Preprint 1118, Department of Mathematics, Utrecht University, 1999, pp. 18.[1]. An update at [2]