On Hadwiger's covering problem in small dimensions

Let Hₙ be the minimal number such that any n-dimensional convex body can be covered by Hₙ translates of interior of that body. Similarly Hₙˢ is the corresponding quantity for symmetric bodies. It is possible to define Hₙ and Hₙˢ in terms of illumination of the boundary of the body using external light sources, and the famous Hadwiger's covering conjecture (illumination conjecture) states that Hₙ=Hₙˢ=2ⁿ. In this note we obtain new upper bounds on Hₙ and Hₙˢ for small dimensions n. Our main idea is to cover the body by translates of John's ellipsoid (the inscribed ellipsoid of the largest volume). Using specific lattice coverings, estimates of quermassintegrals for convex bodies in John's position, and calculations of mean widths of regular simplexes, we prove the following new upper bounds on Hₙ and Hₙˢ: H₅≤ 933, H₆≤ 6137, H₇≤ 41377, H₈≤ 284096, H₄ˢ≤ 72, H₅ˢ≤ 305, and H₆ˢ≤ 1292. For larger n, we describe how the general asymptotic bounds Hₙ≤ 2nnn(ln n+lnln n+5) and Hₙˢ≤ 2ⁿ n(ln n+lnln n+5) due to Rogers and Shephard can be improved for specific values of n.