We consider continuous functions on two-dimensional surfaces satisfying the following conditions: they have a discrete set of local extrema and if a point is not a local extremum, then there exist its neighborhood and a number n ∈ ℕ such that the function restricted to this neighborhood is topologically conjugate to Re z n in a certain neighborhood of zero. Given f : M 2 → ℝ, let Γ K−R (f) be a quotient space of M 2 with respect to its partition formed by the components of level sets of the function f. It is known that the space Γ K−R (f) is a topological graph if M 2 is compact. In the first part of the paper, we introduced the notion of graph with stalks that generalizes the notion of topological graph. For noncompact M 2 , we present three conditions sufficient for Γ K−R (f) to be a graph with stalks. In the second part, we prove that these conditions are also necessary in the case M 2 = ℝ2 . In the general case, one of our conditions is not necessary. We provide an appropriate example.
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E. A. Polulyakh, “Kronrod–Reeb graphs of functions on noncompact two-dimensional surfaces. I” Ukr. Mat. Zh., 67, No. 3, 375–396 (2015); English translation: Ukr. Math. J., 67, No. 3, 431–454 (2015).
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Translated from Ukrains’kyi Matematychnyi Zhurnal, Vol. 67, No. 10, pp. 1398–1408, October, 2015.
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Polulyakh, E.A. Kronrod–Reeb Graphs of Functions on Noncompact Two-Dimensional Surfaces. II. Ukr Math J 67, 1572–1583 (2016). https://doi.org/10.1007/s11253-016-1173-x
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DOI: https://doi.org/10.1007/s11253-016-1173-x