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5 papers
Mathematics
Connected components of spaces of Morse functions with fixed critical points
E. A. Kudryavtseva Lomonosov Moscow State University, Faculty of Mechanics and Mathematics
Abstract:
Let
$M$ be a smooth closed orientable surface and
$F=F_{p,q,r}$ be the space of Morse functions on
$M$ having exactly
$p$ critical points of local minima,
$q\ge1$ saddle critical points, and
$r$ critical points of local maxima, moreover all the points are fixed. Let
$F_f$ be the connected component of a function
$f\in F$ in
$F.$ By means of the winding number introduced by Reinhart (1960), we construct a surjection
$\pi_0(F)\to\mathbb{Z}^{p+r-1}$, in particular
$|\pi_0(F)|=\infty$ and the component
$F_f$ is not preserved under the Dehn twist about the boundary of any disk containing exactly two critical points, exactly one of which is a saddle point. Let
$\mathscr{D}$ be the group of orientation preserving diffeomorphisms of
$M$ leaving fixed the critical points,
$\mathscr{D}^0$ be the connected component of
$\operatorname{id}_M$ in
$\mathscr{D},$ and
$\mathscr{D}_f\subset\mathscr{D}$ the set of diffeomorphisms preserving
$F_f.$ Let
$\mathscr{H}_f$ be the subgroup of
$\mathscr{D}_f$ generated by
$\mathscr{D}^0$ and all diffeomorphisms
$h\in\mathscr{D}$ preserving some functions
$f_1\in F_f,$ and let
$\mathscr{H}_f^\mathrm{abs}$ be its subgroup generated by
$\mathscr{D}^0$ and the Dehn twists about the components of level curves of functions
$f_1\in F_f.$ We prove that $\mathscr{H}_f^\mathrm{abs}\subsetneq\mathscr{D}_f$ if
$q\ge2,$ and construct an epimorphism $\mathscr{D}_f/\mathscr{H}_f^\mathrm{abs}\to\mathbb{Z}_2^{q-1},$ by means of the winding number. A finite polyhedral complex
$K=K_{p,q,r}$ associated to the space
$F$ is defined. An epimorphism
$\mu\colon\pi_1(K)\to\mathscr{D}_f/\mathscr{H}_f$ and finite generating sets for the groups
$\mathscr{D}_f/\mathscr{D}^0$ and
$\mathscr{D}_f/\mathscr{H}_f$ in terms of the 2-skeleton of the complex
$K$ are constructed.
Key words:
Morse functions on a surface, equivalent and isotopic functions, winding number, Dehn twist, admissible diffeomorphism, polyhedral complex.
UDC:
515.164.174,
515.122.55 Received: 18.06.2010