We develop the regularity theory of the spatially homogeneous Boltzmann equation with cut-off and hard potentials (for instance, hard spheres), by (i) revisiting the Lp-theory to obtain constructive bounds, (ii) establishing propagation of smoothness and singularities, (iii) obtaining estimates about the decay of the singularities of the initial datum. Our proofs are based on a detailed study o...

We consider rotations A, B of finite order in SO(3), about axes separated by an angle of restricted type, and attempt to classify the possible group relations between A and B. We show that the relations responsible for the symmetries of Platonic solids have consequences far beyond that simple geometric setting.

We prove that the bicrossed product of two groups is a quotient of the pushout of two semidirect products. A matched pair of groups (H,G,α, β) is deformed using a combinatorial datum (σ, v, r) consisting of an automorphism σ of H , a permutation v of the set G and a transition map r : G → H in order to obtain a new matched pair `