Crosscut criterion for extending isotopies of plane compacta

Let XX be a plane compactum and let h:X×[0,1]Ch:X\times[0,1]\to\mathbb{C} be an isotopy starting at the identity. A crosscut QQ of a complementary domain UU of XX is an arc in UU whose endpoints lie on the boundary of UU and whose interior lies in UU.

Crosscut extension conjecture. The following are equivalent:

  1. hh extends to an isotopy of the entire plane.
  2. For each ε>0\varepsilon>0 there exists δ>0\delta>0 such that, for every complementary domain UU of XX and each crosscut QQ of UU with diam(Q)<δ\operatorname{diam}(Q)<\delta, hh extends to an isotopy
hQ:(XQ)×[0,1]Ch_Q:(X\cup Q)\times[0,1]\to\mathbb{C}

such that, for all t[0,1]t\in[0,1],

diam(ht(Q))<ε.\operatorname{diam}(h^t(Q))<\varepsilon.

This would remove the uniform-perfectness assumption used in the proved extension theorem and give a necessary and sufficient criterion for extending isotopies of arbitrary plane compacta. The source does not provide evidence that the conjecture has been resolved.

Sources & referencesView supporting material

Primary source

L. C. Hoehn, L. G. Oversteegen and E. D. Tymchatyn, “Extension of isotopies in the plane”, arXiv:1808.05601 (2018).

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