ON THE COST OF OBSERVABILITY IN SMALL TIMES FOR THE ONE-DIMENSIONAL HEAT EQUATION

We aim at presenting a new estimate on the cost of observability in small times of the one-dimensional heat equation, which also provides a new proof of observability for the one-dimensional heat equation. Our proof combines several tools. First, it uses a Carleman-type estimate borrowed from our previous work (SIAM J. Control Optim. 56:3 (2018), 1692-1715), in which the weight function is derived from the heat kernel and which is therefore particularly easy. We also use explicit computations in the Fourier domain to compute the high-frequency part of the solution in terms of the observations. Finally, we use the Phragmen-Lindelof principle to estimate the low-frequency part of the solution. This last step is done carefully with precise estimations coming from conformal mappings.