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Gruber P. Convex and Discrete Geometry

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316 <strong>Convex</strong> Polytopes<br />

The Coefficients of the Ehrhart Polynomial<br />

Based on Barvinok’s [78, 79] method for counting lattice points, DeLoera,<br />

Hemmecke, Tauzer <strong>and</strong> Yoshida [254] developed an efficient algorithm to calculate<br />

the coefficients of Ehrhart polynomials.<br />

19.2 Theorems of Pick, Reeve <strong>and</strong> Macdonald on Volume <strong>and</strong> Lattice Point<br />

Enumerators<br />

Can the volume of a lattice polytope P be calculated in terms of the number of lattice<br />

points in P, in the relative interior of P <strong>and</strong> on the relative boundary of P, that is, in<br />

terms of L(P), L o (P) <strong>and</strong> L b (P)?<br />

A nice result of Pick [801] says that, for a Jordan lattice polygon P in E 2 ,<br />

(1) A(P) = L(P) − 1 2 Lb (P) − 1,<br />

where A st<strong>and</strong>s for area. For this result there are many proofs, variants <strong>and</strong> extensions<br />

known, as a look into the American Mathematical Monthly, the Mathematics Teacher<br />

<strong>and</strong> similar journals shows. We mention the far-reaching generalizations of Hadwiger<br />

<strong>and</strong> Wills [470] <strong>and</strong> Grünbaum <strong>and</strong> Shephard [456].<br />

There is no direct generalization of the formula (1) to higher dimension as can<br />

be seen from the simplices Sn = conv{o,(1, 0, 0), (0, 1, 0), (1, 1, n)} in E 3 .<br />

V (Sn) = n<br />

3! , while L(Sn) = L b (Sn) = 4, L o (Sn) = 0forn ∈ N.<br />

Reeve [825, 826] (d = 3) <strong>and</strong> Macdonald [675] (general d) were able to express the<br />

volume of a (general) lattice polytope P in terms of L(P), L b (P), L(2P),<br />

L b (2P),...,L � (d − 1)P � , L b� (d − 1)P � .<br />

This section contains the proofs of Pick’s theorem <strong>and</strong> of results of Reeve <strong>and</strong><br />

Macdonald in the special case of convex lattice polytopes.<br />

For more information, see the references given earlier <strong>and</strong> the references at the<br />

beginning of Sect. 19.<br />

Pick’s Lattice Point Theorem in E 2<br />

A Jordan lattice polygon is a solid polygon in E 2 bounded by a closed Jordan polygonal<br />

curve in E 2 , all vertices of which are points of Z 2 .LetJ Z 2 be the family of<br />

all Jordan lattice polygons. Clearly, the lattice point enumerators extend to J Z 2.The<br />

following result is due to Pick [801].<br />

Theorem 19.2. Let P ∈ J Z 2. Then (Fig. 19.1)<br />

(1) A(P) = L(P) − 1 2 Lb (P) − 1.<br />

Proof. We first show that<br />

(2) The expression M = L − 1 2 Lb − 1issimply additive on J Z 2.

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