The cost of controlling weakly degenerate parabolic equations by boundary controls

We consider a one-dimensional degenerate parabolic equation, controlled by a boundary force acting at the degeneracy point x = 0. We study the reachable targets at some given time T, studying the influence of the degeneracy parameter α ∈ [0, 1). First we obtain precise upper and lower bounds for the null controllability cost, proving that the cost blows up rationnally as α → 1− and exponentially fast when T → 0+. Next, thanks to the special structure of the eigenfunctions of the problem, we investigate and obtain (partial) results concerning the structure of the reachable states. Our approach is based on the moment method developed by Fattorini and Russell. To achieve our goals, we extend some of their general re- sults concerning biorthogonal families, using complex analysis techniques de- velopped by Seidman, Guichal, Tenenbaum-Tucsnak, and Lissy.