We derive tight expressions for the maximum number of k-faces, k=0,...,d-1, of the Minkowski sum, P_1+...+P_r, of r convex d-polytopes P_1,...,P_r in R^d, where d >= 2 and r < d, as a (recursively defined) function on the number of vertices of the polytopes. Our results coincide with those recently proved by Adiprasito and Sanyal [1]. In contrast to Adiprasito and Sanyal's approach, which uses tools from Combinatorial Commutative Algebra, our approach is purely geometric and uses basic notions such as f- and h-vector calculus, stellar subdivisions and shellings, and generalizes the methodology used in [10] and [9] for proving upper bounds on the f-vector of the Minkowski sum of two and three convex polytopes, respectively. The key idea behind our approach is to express the Minkowski sum P_1+...+P_r as a section of the Cayley polytope C of the summands; bounding the k-faces of P_1+...+P_r reduces to bounding the subset of the (k+r-1)-faces of C that contain vertices from each of the r polytopes. We end our paper with a sketch of an explicit construction that establishes the tightness of the upper bounds.

We derive tight expressions for the maximum number of k-faces, 0≤k≤d-1, of the Minkowski sum, P1+P2+P3, of three d-dimensional convex polytopes P1, P2 and P3 in Rd, as a function of the number of vertices of the polytopes, for any d≥2. Expressing the Minkowski sum as a section of the Cayley polytope C of its summands, counting the k-faces of P1+P2+P3 reduces to counting the (k+2)-faces of C which meet the vertex sets of the three polytopes. In two dimensions our expressions reduce to known results, while in three dimensions, the tightness of our bounds follows by exploiting known tight bounds for the number of faces of r d-polytopes in Rd, where r≥d. For d≥4, the maximum values are attained when P1, P2 and P3 are d-polytopes, whose vertex sets are chosen appropriately from three distinct d-dimensional moment-like curves.

We derive tight bounds for the maximum number of k-faces, 0 ≤ k ≤ d − 1, of the Minkowski sum, P1 ⊕ P2, of two d-dimensional convex polytopes P1 and P2, as a function of the number of vertices of the polytopes. For even dimensions d ≥ 2, the maximum values are attained when P1 and P2 are cyclic d-polytopes with disjoint vertex sets. For odd dimensions d ≥ 3, the maximum values are attained when P1 and P2 are ⌊d/2⌋-neighborly d-polytopes, whose vertex sets are chosen appropriately from two distinct d-dimensional moment-like curves.

Given a set Σ of spheres in Ed, with d≥3 and d odd, having a fixed number of m distinct radii ρ1,ρ2,...,ρm, we show that the worst-case combinatorial complexity of the convex hull CHd(Σ) of Σ is Θ(Σ{1≤i≠j≤m}ninj⌊ d/2 ⌋), where ni is the number of spheres in Σ with radius ρi. Our bound refines the worst-case upper and lower bounds on the worst-case combinatorial complexity of CHd(Σ) for all odd d≥3.

Let $\Phi$ be a finite root system with corresponding reflection group W, and let m be a nonnegative integer. We consider the generalized cluster complex $\Delta^m(\Phi)$ defined by Fomin and Reading and the poset $NC_{(m)}(W)$ of m-divisible noncrossing partitions defined by Armstrong. We give a characterization of the faces of $\Delta^m(\Phi)$ in terms of $NC_{(m)}(W)$, generalizing that of Brady and Watt given in the case $m=1$. Making use of this, we give a case free proof of a conjecture of Chapoton and Armstrong, which relates a certain refined face count of $\Delta^m(\Phi)$ with the Möbius function of $NC_{(m)}(W)$.