Smooth Invariants of Focus-Focus Singularities / *Izosimov A.M.* // Vestnik Moskovskogo Universiteta. Seriya 1. Matematika. Mekhanika. 2011. № 4. P. 59-61
[Moscow Univ. Math. Bulletin. Vol. 72, N 2, 2017. P. 0].

We consider focus-focus singularities of integrable Hamiltonian
systems with two degrees of freedom. It is known that if two singularities
of this kind have the same number of singular points, then they are
fiber-wise homeomorphic. However, it may happen that this homeomorphism
cannot be made smooth. An invariant is constructed, which makes
it possible to classify focus-focus singularities up to a C^{1}-diffeomorphism.

*Key words*:
Hamiltonian systems, singularities, integrability.