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Let $M^3$ be a closed orientable smooth manifold and let $g_t$ be a (smooth) $1$-parameter family of Riemannian metrics on $M$, $t \in \mathbb{R}$. Let $P \subset M$ a fixed closed orientable embedded surface with the property that $P$ is minimal with respect to the metrics $g_t$. Finally, let $\Sigma_t$, $t \in \mathbb{R}$, be a (smooth) $1$-parameter family of embedded minimal normal graphs over $P$ (with respect to $g_t$); this means that there exists a smooth family of maps $f_t : P \to \mathbb{R}$ such that

$$\Sigma_t = \{ \operatorname{exp}_p(f_t(p)N_t(p)) : p \in P \}$$

where $N_t$ is a unit normal for $P$ with respect to $g_t$ and each $\Sigma_t$ is minimally embedded in $(M, g_t)$.

My question: if $\Sigma_0 = P$ and $\Sigma_t \neq P$ for $t \neq 0$, how does the number of connected components of $Q_t := P \setminus \Sigma_t$ change with $t \neq 0$? Is it constant?

My intuition tells me that if the number of connected components in $Q_t$ changes at $t = t_0$ then $\Sigma_{t_0}$ and $P$ must be tangent, with one surface locally on one side of the other, which is forbidden by the maximum principle. Is this reasoning correct?

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  • $\begingroup$ If you forget about the minimality for a moment, what is the example that you have in mind? I can't picture a family of embeddings where this would change - the $(\Sigma_t)$ are isotopic after all, no? If you relax the hypotheses to allow immersions, a modification of the starfish example of Pitts seems promising. This would give a minimal family where one goes from two to three components. $\endgroup$ Commented Oct 20, 2021 at 6:58
  • $\begingroup$ I was thinking of a bifucartion branch of minimal tori about a $2$-torus, that may oscilate but never change the aforementioned quantity. $\endgroup$ Commented Oct 20, 2021 at 19:23
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    $\begingroup$ I don't completely follow, but that may just be me. Perhaps you could sketch the example with the tori in a bit more detail in your question? $\endgroup$ Commented Oct 20, 2021 at 21:36
  • $\begingroup$ The example I have in mind is kind of theoretical, just like I described. What happens is that all surfaces involved are tori; $M = S \times \mathbb{S}^1$, $g_t = g + t^2 d\theta^2$ and $P$ is a product of a simple closed geodesic of $S$ with $\mathbb{S}^1$. $\endgroup$ Commented Oct 21, 2021 at 1:11

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