From the Sphere to the Grassmannian: Continuous Brascamp--Lieb Inequalities via Measure Approximation and Heat Flow

We study continuous Brascamp–Lieb inequalities associated with isotropic measures on the sphere $S^{n-1}$ and on the Grassmannian $G(n,k)$. We give two proofs of the continuous Grassmannian inequality. The first proof approximates the continuous isotropic measure by discrete isotropic measures and then applies the equal-rank geometric Brascamp–Lieb inequality. In the rank-one case, the construction of the discrete isotropic measures follows from Barthe’s work, which is standard, whereas in the higher-rank case the construction of finite approximate isotropic measures is given by a Tchakaloff-type theorem. Weak convergence, geometric Brascamp–Lieb inequality and Fatou’s lemma then yield the continuous inequalities. The second proof generalizes the heat-semigroup preservation method of Barthe and Huet to isotropic measures on $G(n,k)$. We prove a heat-semigroup preservation theorem in the Grassmannian setting and show that this theorem yields a Grassmannian Brascamp–Lieb inequality. At the end, we also give a sketch for the mixed-rank case of the continuous Brascamp–Lieb inequality.