  George Birkhoff proved in 1912 that the number of proper colorings of a
finite graph G with n colors is a polynomial in n, called the chromatic
polynomial of G. Read conjectured in 1968 that for any graph G, the sequence of
absolute values of coefficients of the chromatic polynomial is unimodal: it
goes up, hits a peak, and then goes down. Read's conjecture was proved by June
Huh in a 2012 paper making heavy use of methods from algebraic geometry. Huh's
result was subsequently refined and generalized by Huh and Katz, again using
substantial doses of algebraic geometry. Both papers in fact establish
log-concavity of the coefficients, which is stronger than unimodality.
  The breakthroughs of Huh and Huh-Katz left open the more general Rota-Welsh
conjecture where graphs are generalized to (not necessarily representable)
matroids and the chromatic polynomial of a graph is replaced by the
characteristic polynomial of a matroid. The Huh and Huh-Katz techniques are not
applicable in this level of generality, since there is no underlying algebraic
geometry to which to relate the problem. But in 2015 Adiprasito, Huh, and Katz
announced a proof of the Rota-Welsh conjecture based on a novel approach
motivated by but not making use of any results from algebraic geometry. The
authors first prove that the Rota-Welsh conjecture would follow from
combinatorial analogues of the Hard Lefschetz Theorem and Hodge-Riemann
relations in algebraic geometry. They then implement an elaborate inductive
procedure to prove the combinatorial Hard Lefschetz Theorem and Hodge-Riemann
relations using purely combinatorial arguments.
  We will survey these developments.
