McMullen is known for his work in
polyhedral combinatorics and
discrete geometry, and in particular for proving what was then called the upper bound conjecture and now is the
upper bound theorem. This result states that
cyclic polytopes have the maximum possible number of faces among all polytopes with a given dimension and number of vertices. McMullen also formulated the g-conjecture, later the
g-theorem of
Louis Billera, Carl W. Lee, and
Richard P. Stanley, characterizing the
f-vectors of
simplicial spheres. The
McMullen problem is an unsolved question in discrete geometry named after McMullen, concerning the number of points in
general position for which a
projective transformation into
convex position can be guaranteed to exist. It was credited to a private communication from McMullen in a 1972 paper by David G. Larman. He is also known for his 1960s drawing, by hand, of a 2-dimensional representation of the Gosset polytope
421, the vertices of which form the vectors of the
E8 root system.{{cite web ==Awards and honours==