The convex hull of the roots of a classical root lattice is called a root polytope. We determine explicit unimodular triangulations of the boundaries of the root polytopes associated to the root lattices An, Cn, and Dn, and we compute their f -and h-vectors. This leads us to recover formulae for the growth series of these root lattices, which were first conjectured by Conway, Mallows, and Sloane and Baake and Grimm and were proved by Conway and Sloane and Bacher, de la Harpe, and Venkov. We also prove the formula for the growth series of the root lattice Bn, which requires a modification of our technique.
Introduction.A lattice L is a discrete subgroup of R n for some n ∈ Z >0 . The rank of a lattice is the dimension of the subspace spanned by the lattice. We say that a lattice L is generated as a monoid by a finite collection of vectors M = {a 1 , . . . , a r } if each u ∈ L is a nonnegative integer combination of the vectors in M. For convenience, we often write the vectors from M as columns of a matrix M ∈ R n×r , and to make the connection between L and M more transparent, we refer to the lattice generated by M as L M . The word length of u with respect to M, denoted w(u), is min( c i ) taken over all expressions u = c i a i with c i ∈ Z ≥0 . The growth function S(k) counts the number of elements u ∈ L with word length w(u) = k with respect to M. We define the growth series to be the generating function G(x) := k≥0 S(k) x k . It is a