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Gruber P. Convex and Discrete Geometry

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7 Valuations 111<br />

the very readable popular article of Rota [858]. Special valuations in the context of<br />

polytopes <strong>and</strong> lattice polytopes will be dealt with in Sects. 16.1 <strong>and</strong> 19.4.<br />

7.1 Extension of Valuations<br />

Given a valuation on a family of sets, the problem arises to extend this valuation to<br />

larger families of sets, for example to the (algebraic) lattice of sets generated by the<br />

given family.<br />

This section contains the inclusion–exclusion formula <strong>and</strong> the extension results<br />

of Voll<strong>and</strong>, rediscovered by Perles <strong>and</strong> Sallee in a more abstract form, <strong>and</strong> of<br />

Groemer.<br />

The Inclusion–Exclusion Formula<br />

Let L, or more precisely, 〈L, ∩, ∪〉, be a lattice of sets, where the lattice operations<br />

are the ordinary intersection <strong>and</strong> union. Given a valuation φ : L → R, iterating the<br />

relation<br />

φ(C ∪ D) + φ(C ∩ D) = φ(C) + φ(D) for C, D ∈ L,<br />

easily leads to the equality,<br />

(1) φ(C1 ∪···∪Cm) = �<br />

φ(Ci) − �<br />

φ(Ci ∩ C j) +<br />

+ �<br />

i< j

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