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Gruber P. Convex and Discrete Geometry

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112 <strong>Convex</strong> Bodies<br />

sets of S. Ifφ can be extended to a valuation on L(S), then this extension satisfies<br />

the inclusion–exclusion formula on L, as seen before. This shows that, in particular,<br />

the original valuation φ on S must satisfy the inclusion–exclusion formula for an<br />

intersectional family on S:<br />

(3) φ(C1 ∪···∪Cm) = �<br />

φ(Ci) − �<br />

φ(Ci ∩ C j) +−···<br />

i<br />

= �<br />

(−1) |I |−1 φ(CI ),forC1,...,Cm, C1 ∪···∪Cm ∈ S.<br />

I<br />

Thus (3) is a necessary condition for a valuation φ on S to be extendible to a valuation<br />

on L(S). Surprisingly, this simple necessary condition for extension is also sufficient,<br />

as the following first extension theorem of Voll<strong>and</strong> [1011] shows; see also Perles <strong>and</strong><br />

Sallee [792].<br />

Theorem 7.1. Let S be an intersectional family of sets <strong>and</strong> φ : S → R a valuation.<br />

Then the following claims are equivalent:<br />

i< j<br />

(i) φ satisfies the inclusion–exclusion formula on S.<br />

(ii) φ has a unique extension to a valuation on L(S).<br />

The proof follows Voll<strong>and</strong>, but requires filling a gap.<br />

Proof. Since the implication (ii)⇒(i) follows from what was said above, it is sufficient<br />

to prove that<br />

(i)⇒(ii) In a first step it will be shown that<br />

(4) �<br />

(−1) |I |−1 φ(CI ) = �<br />

(−1) |J|−1 φ(DJ )<br />

I<br />

J<br />

for C1,...,Cm, D1,...,Dn ∈ S, where C1 ∪···∪Cm = D1 ∪···∪ Dn.<br />

Since C1 ∪···∪Cm = D1 ∪···∪ Dn,<br />

�<br />

(−1) |I |−1 φ(CI ) = �<br />

(−1) |I |−1 φ � CI ∩ (D1 ∪···∪ Dn) �<br />

I<br />

I<br />

= �<br />

(−1) |I |−1 φ � (CI ∩ D1) ∪···∪(CI ∩ Dn) �<br />

I<br />

= �<br />

(−1)<br />

I<br />

= �<br />

(−1)<br />

J<br />

�<br />

|I |−1<br />

(−1) |J|−1 φ(CI ∩ DJ )<br />

J<br />

�<br />

|J|−1<br />

I<br />

(−1) |I |−1 φ(DJ ∩ CI ) =···= �<br />

(−1) |J|−1 φ(DJ ),<br />

concluding the proof of (4).<br />

Define a function φ : L(S) → R by<br />

(5) φ(C1 ∪···∪Cm) = �<br />

(−1) |I |−1 φ(CI ) for C1,...,Cm ∈ S,<br />

I<br />

J

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