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Let $S$ be a smooth surface and $\gamma_1, \gamma_2$ be two transversal simple closed curves on it. Suppose moreover that there exists a simple closed curve $\gamma_1'$ on $S$ isotopic to $\gamma_1$ and such that $\#(\gamma_1\cap \gamma_2)>\#(\gamma_1'\cap \gamma_2)$.

Question. Is it true that there is a disk on $S\setminus (\gamma_1\cup\gamma_2)$ whose boundary is composed of one arc of $\gamma_1$ and one arc of $\gamma_2$?

Note that in case such a disk exists, one can construct an isotopy of $\gamma_1$ that would decrease the number of intersections of $\gamma_1$ with $\gamma_2$ by two.

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    $\begingroup$ Yes. This is discussed in Farb-Margalit’s “Primer on mapping class groups”, where it is called the bigon criterion. $\endgroup$
    – Andy Putman
    Commented Sep 12, 2020 at 23:41
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    $\begingroup$ This MSE post may help: Proof of the bigon criterion, especially Lee Moser's answer. $\endgroup$
    – Joseph O'Rourke
    Commented Sep 12, 2020 at 23:57

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This is also proved as Lemma 3.1 in Joel Hass and Peter Scott, Intersections of curves on surfaces, Israel Journal of Mathematics 51 (1985), 90–120. https://doi.org/10.1007/BF02772960

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