Optimal Gagliardo-Nirenberg interpolation inequality for rearrangement invariant spaces

Abstract: We prove optimality of the Gagliardo-Nirenberg inequality $$ |\nabla u|_{X}\lesssim|\nabla^2 u|_Y^{1/2}|u|_Z^{1/2}, $$ where $Y, Z$ are rearrangement invariant Banach function spaces and $X=Y^{1/2}Z^{1/2}$ is the Calder\'on--Lozanovskii space. By optimality, we mean that for a certain pair of spaces on the right-hand side, one cannot reduce the space on the left-hand, remaining in the class of rearrangement invariant spaces. The optimality for the Lorentz and Orlicz spaces is given as a consequence, exceeding previous results. We also discuss pointwise inequalities, their importance and counterexample prohibiting an improvement.