14.02.2013 Views

Gruber P. Convex and Discrete Geometry

Gruber P. Convex and Discrete Geometry

Gruber P. Convex and Discrete Geometry

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

8 The Brunn–Minkowski Inequality 145<br />

This, in turn, implies that<br />

tC(s)<br />

(9)<br />

V (C) 1 =<br />

d<br />

tD(s)<br />

V (D) 1 + const for 0 ≤ s ≤ 1,<br />

d<br />

where we have used the continuity of tC, tD for 0 ≤ s ≤ 1. Since o is the centroid<br />

of C, Fubini’s theorem, (4) <strong>and</strong> (5) show that<br />

�<br />

0 = u · xdx=<br />

=<br />

C<br />

�1<br />

0<br />

�βC<br />

αC<br />

t v � C ∩ H(t) � dt =<br />

�<br />

tC(s)vC tC(s) � t ′ C (s) ds =<br />

<strong>and</strong> similarly for D.<br />

�1<br />

0<br />

�βC<br />

αC<br />

t vC(t) dt<br />

tC(s) ds V(C),<br />

The constant in (9) is thus 0. Using support functions for C <strong>and</strong> D, Proposition (9)<br />

implies that<br />

h D(u) = βD = tD(1) =<br />

�<br />

V (D)<br />

� 1<br />

d<br />

tC(1) =<br />

V (C)<br />

Since u ∈ S d−1 was arbitrary, it follows that<br />

D =<br />

�<br />

V (D)<br />

� 1<br />

d<br />

C.<br />

V (C)<br />

�<br />

V (D)<br />

� 1<br />

d<br />

βC =<br />

V (C)<br />

�<br />

V (D)<br />

� 1<br />

d<br />

hC(u).<br />

V (C)<br />

This settles the equality case. ⊓⊔<br />

Other Common Versions of the Classical Brunn–Minkowski Inequality<br />

The above version of the Brunn–Minkowski inequality readily yields the following<br />

results.<br />

Theorem 8.2. Let C, D ∈ C. Then<br />

V � (1 − λ)C + λD � 1 d ≥ (1 − λ)V (C) 1 d + λV (D) 1 d for 0 ≤ λ ≤ 1,<br />

where there is equality for some λ with 0

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!