Operator Monotonicity and Convexity
Vjosa Blakaj Homework of Topic 5
May 19, 2018
Problem 1.
1. Show that the square root function is matrix monotone in a different way from that presented in class.
2. Lowener Theorem tells us that the function f(x) = xt for every 0 ≤ t ≤ 1 is matrix monotone. Does the monotonicity still holds for t >1?
Problem 2.
Show that the functionf(x) := exp(x) is not matrix monotone.
Hint: Use the divided difference matrix.
Problem 3.
Show that the map (A, B)7→M0(A, B) on Hn+ x Hn+ is jointly concave.
Hint: Use the identity
M0(A, B) = max{C |
A C
C B
≥0} (1)
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