From internal to pointwise control for the 1D heat equation and minimal control time

Our goal is to study controllability and observability properties of the 1D heat equation with internal control (or observation) set ${\omega }_{\epsilon }=(x_0-\epsilon ,x_0+\epsilon )$, in the limit $\epsilon \to 0$, where $x_0\in (0,1)$. It is known that depending on arithmetic properties of $x_0$, there may exist a minimal time $T_0$ of pointwise control at $x_0$ of the heat equation. Besides, for any $\epsilon$ fixed, the heat equation is controllable with control set ${\omega }_{\epsilon }$ in any time $T>0$. We relate these two phenomena. We show that the observability constant on ${\omega }_{\epsilon }$ does not converge to $0$ as $\epsilon \to 0$ at the same speed when $T>T_0$ (in which case it is comparable to ${\epsilon }^{1\mathrm{/}2}$) or $T<T_0$ (in which case it converges faster to $0$). We also describe the behavior of optimal $L^2$ null-controls on ${\omega }_{\epsilon }$ in the limit $\varepsilon \rightarrow 0$.