Integral polytope

An integral polytope is a polytope where all vertices are at integer coordinates. Examples include the cube, all associahedra, and the Birkhoff polytopes.

Pick's theorem and Ehrhart polynomials
Integral polygons that lack self-intersections or holes are subject to a simple formula known as Pick's theorem that relate the area of the polygon A, the number of lattice points on the boundary b, and the number of lattice points in the interior i. The theorem states that $$A = i + b/2 - 1$$.

Attempting to generalize Pick's theorem to three dimensions or more requires extra work, as shown by John Reeve in 1957. Consider a tetrahedron whose vertices are located at (0, 0, 0), (1, 0, 0), (0, 1, 0), and (1, 1, r) for integer r > 0. The volume of this tetrahedron is r/6, but the number of interior and boundary lattice points does not change with r, so the tetrahedron's volume cannot be a function of i and b. These tetrahedra are known as Reeve tetrahedra.

The proper generalization to any number of dimensions is given by Ehrhart polynomials. For an integral polytope P, let L(t) be the number of lattice points contained inside P if each vertex in P is scaled from the origin by a factor of t. Then L(t) is a polynomial in t with rational coefficients.