Round function

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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[edit]

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 rank Hess}(G)=1 at the tagged circles, making the critical point degenerate, that is, showing that the critical points are not isolated.

Round complexity[edit]

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[edit]

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