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Gruber P. Convex and Discrete Geometry

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144 <strong>Convex</strong> Bodies<br />

C ∩ H(tC )<br />

C<br />

D ∩ H(tD)<br />

D<br />

C ∩ H(tC ) + D ∩ H(tD)<br />

C + D<br />

Fig. 8.1. Proof of the Brunn–Minkowski theorem; there is no misprint in the figure for C + D<br />

=<br />

�1<br />

0<br />

1<br />

�<br />

≥<br />

0<br />

1<br />

�<br />

≥<br />

0<br />

1<br />

�<br />

≥<br />

0<br />

v � (C + D) ∩ H � tC+D(s) �� t ′ C+D (s) ds<br />

v � C ∩ H � tC(s) � + D ∩ H � tD(s) �� t ′ C+D (s) ds<br />

� � � 1 � � 1<br />

vC tC(s) d−1 + v D tD(s) d−1 � �<br />

d−1<br />

vC<br />

� 1<br />

V (C) d + V (D) 1 �dds � 1<br />

d = V (C) d + V (D) 1 �d. d<br />

V (C)<br />

� � +<br />

tC(s)<br />

V (D)<br />

�<br />

tD(s) �<br />

�<br />

ds<br />

In the second part of the proof, we assume that equality holds in (1). By translating<br />

C <strong>and</strong> D, if necessary, we may suppose that o is the centroid of both C <strong>and</strong><br />

D. Letu ∈ S d−1 . Since, by assumption, there is equality in (1), we have equality<br />

throughout (8). Thus, in particular (see Fig. 8.1),<br />

�<br />

vC tC(s) �<br />

V (C) d−1<br />

d<br />

= v �<br />

D tD(s) �<br />

V (D) d−1<br />

d<br />

by (2). An application of (5) then shows that<br />

t ′ C (s)<br />

V (C) 1 d<br />

= t′ D (s)<br />

V (D) 1 d<br />

for 0 < s < 1,<br />

for 0 < s < 1.<br />

v D

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