Weakly non-collapsed RCD spaces are strongly non-collapsed

Abstract

Abstract We prove that any weakly non-collapsed RCD space is actually non-collapsed, up to a renormalization of the measure. This confirms a conjecture raised by De Philippis and the second named author in full generality. One of the auxiliary results of independent interest that we obtain is about the link between the properties tr ⁡ ( Hess ⁡ f ) = Δ ⁢ f \operatorname{tr}(\operatorname{Hess}f)=\Delta f on U ⊆ X U\subseteq{\mathsf{X}} for every 𝑓 sufficiently regular, m = c ⁢ H n \mathfrak{m}=c\mathscr{H}^{n} on U ⊆ X U\subseteq{\mathsf{X}} for some c > 0 c>0 , where U ⊆ X U\subseteq{\mathsf{X}} is open and 𝖷 is a – possibly collapsed – RCD space of essential dimension 𝑛.