We study $n$-vertex $d$-dimensional polytopes with at most one nonsimplex facet with, say, $d+s$ vertices, called almost simplicial polytopes. We provide tight lower and upper bound theorems for these polytopes as functions of $d,n$, and $s$, thus generalizing the classical Lower Bound Theorem by Barnette and the Upper Bound Theorem by McMullen, which treat the case where $s=0$. We characterize the minimizers and provide examples of maximizers for any $d$. Our construction of maximizers is a generalization of cyclic polytopes, based on a suitable variation of the moment curve, and is of independent interest.

We investigate how the Minkowski sum of two polytopes affects their graph and, in particular, their diameter. We show that the diameter of the Minkowski sum is bounded below by the diameter of each summand and above by, roughly, the product between the diameter of one summand and the number of vertices of the other. We also prove that both bounds are sharp. In addition, we obtain a result on polytope decomposability. More precisely, given two polytopes $P$ and $Q$, we show that $P$ can be written as a Minkowski sum with a summand homothetic to $Q$ if and only if $P$ has the same number of vertices as its Minkowski sum with $Q$.

Let G = (V,E) be a simple undirected graph.A forest F ⊆ E of G is said to be clique-connecting if each tree of F spans a clique of G.This paper adresses the clique-connecting forest polytope.First we give a formulation and a polynomial time separation algorithm. Then we show that the nontrivial nondegenerate facets of the stable set polytope are facets of the clique-connecting polytope.Finally we introduce a family of rank inequalities which are facets, and which generalize the clique inequalities.

In this paper, we study the Steiner 2-edge connected subgraph polytope.We introduce a large class of valid inequalities for this polytope calledthe generalized Steiner F-partition inequalities, that generalizesthe so-called Steiner F-partition inequalities. We show that theseinequalities together with the trivial and the Steiner cutinequalities completely describe the polytope on a class of graphsthat generalizes the wheels. We also describe necessary conditions forthese inequalities to be facet defining, and as a consequence, weobtain that the separation problem over the Steiner 2-edge connectedsubgraph polytope for that class of graphs can be solved in polynomialtime. Moreover, we discuss that polytope in the graphs that decomposeby 3-edge cutsets. And we show that the generalized SteinerF-partition inequalities together with the trivial and the Steinercut inequalities suffice to describe the polytope in a class of graphsthat generalizes the class of Halin graphs when the terminals have aparticular disposition. This generalizes a result of Barahona andMahjoub [4] for Halin graphs. This also yields apolynomial time cutting plane algorithm for the Steiner 2-edgeconnected subgraph problem in that class of graphs.

Petrie polygons, especially as they arise in the study of regular polytopes and Coxeter groups, have been studied by geometers and group theorists since the early part of the twentieth century. An open question is the determination of which polyhedra possess Petrie polygons that are simple closed curves. The current work explores combinatorial structures in abstract polytopes, called Petrie schemes, that generalize the notion of a Petrie polygon. It is established that all of the regular convex polytopes and honeycombs in Euclidean spaces, as well as all of the Grünbaum–Dress polyhedra, possess Petrie schemes that are not self-intersecting and thus have Petrie polygons that are simple closed curves. Partial results are obtained for several other classes of less symmetric polytopes.