260 Elements of Nonlinear Analysis
(iii) If K is not compact, assume that there exist a nonempty compact convex
subset B of K and a nonempty compact subset D of K such that for
each x ∈ K \ D there exists ˜y ∈ B such that ˜y ∈ P (x).
Then, there exists ¯x ∈ K such that ¯x ∈ Q(¯x).
Theorem B.5. Let K be a nonempty convex subset of R
n
and P : K → 2
K
a set-valued map. Assume that the following conditions hold:
(i) for all x ∈ K, P (x) is convex;
(ii) for each finite subset A of K and for a ll y ∈ co(A), P
−1
(y) ∩ co(A) is
open in co(A);
(iii) for each finite subset A of K and all x, y ∈ co(A) and every sequence
{x
m
} in K converging to x such that λy + (1 − λ)x /∈ P (x
m
) for all
m ∈ N and all λ ∈ [0, 1], we have y /∈ P (x);
(iv) there exist a nonempty compact subset D of K a nd an element ˜y ∈ D
such that ˜y ∈ P (x) for all x ∈ K \ D;
(v) for all x ∈ D, P (x) is nonempty.
Then, there exists ˆx ∈ K such that ˆx ∈ P (ˆx).
Theorem B .6 (Kakutani). [119] Let K be a nonempty compact convex sub-
set of R
n
and P : K → 2
K
be a set-valued map such that for each x ∈ K,
P (x) is nonempty, compact, and conve x. Then P has a fixed point, that is,
there exists ¯x ∈ K such that ¯x ∈ P (¯x).
Theorem B.7 (Kneser). [131] Let K be a nonempty convex subset of R
n
and
D be a nonempty compact convex subset of R
m
. Suppose that f : K × D →
R is lower semicontinuous in the second argument and concave in the first
argument. Then
min
y∈D
sup
x∈K
f(x, y) = sup
x∈K
min
y∈Y
f(x, y).
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