In topology and in calculus, a round function is a scalar function , over a manifold , whose critical points form one or several connected components, each homeomorphic to the circle , also called critical loops. They are special cases of Morse-Bott functions.
For example, let be the torus. Let
Then we know that a map
is a parametrization for almost all of . Now, via the projection we get the restriction
is a function whose critical sets are determined by
this is if and only if .
These two values for give the critical sets
which represent two extremal circles over the torus .
Observe that the Hessian for this function is
which clearly it reveals itself as rank of equal to one at the tagged circles, making the critical point degenerate, that is, showing that the critical points are not isolated.
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.