Humboldt-Universit¨at zu Berlin Institut f¨ur Mathematik

C. Wendl, S. Dwivedi, L. Upmeier zu Belzen

### Funktionalanalysis

WiSe 2020–21

### Problem Set 4

Due: Thursday, 3.12.2020 (22pts total)

Problems marked with p˚q will be graded. Solutions may be written up in German or English and should be submitted electronically via the moodle before the ¨Ubung on the due date. For problems without p˚q, you do not need to write up your solutions, but it is highly recommended that you think through them before the next Tuesday lecture. You may also use the results of those problems in your written solutions to the graded problems.

Convention:Unless otherwise stated, you can assume in every problem thatpX, µqis an
arbitrary measure space and functions inL^{p}pXq:“L^{p}pX, µqtake values in a fixed finite-
dimensional inner product spacepV,x, yqover a fieldKwhich is eitherRorC. Whenever
X is a subset of R^{n}, you can also assume by default that µis the Lebesgue measurem.

Problem 1 p˚q

Assume 1†p, q †8with ^{1}_{p}` ^{1}_{q} “1. Prove:

(a) For any closed subspace E Ä L^{p}pXq with E ‰ L^{p}pXq, there exists a function g P
L^{q}pXqzt0u that satisfies≥

Xxg, fydµ“0 for everyf PE.

Hint: SinceL^{p}pXqis uniformly convex, there exists a closest point inE to any given
point in L^{p}pXqzE. [6pts]

(b) A linear subspace E ÄL^{p}pXqis dense if and only if every bounded linear functional

⇤:L^{p}pXq ÑK that vanishes on E is trivial. [3pts]

Comment: The result of this problem is often used in applications and cited as a con-
sequence of the Hahn-Banach theorem, which implies a similar result for subspaces of
arbitrary Banach spaces. However, the uniform convexity of L^{p}pXqfor 1† p†8 makes
the use of the Hahn-Banach theorem (which relies on the axiom of choice) unnecessary
in this setting. You should not use it in your solution either, since we have not proved it yet.

Problem 2

Show that iff PL^{8}pXqsatisfies |f|†}f}L^{8} almost everywhere, then
ˇˇ

ˇˇ ª

Xxg, fydµ ˇˇ

ˇˇ†}g}L^{1}¨ }f}L^{8} for every gPL^{1}pXqzt0u,

i.e. there is strict inequality. Give an example f PL^{8}pr0,1sq satisfying this condition.

Hint: What can you say about ≥

Xpc´ |f|q|g|dµ if|f|†c almost everywhere?

Comment: The Hahn-Banach theorem implies that for every nontrivial element x in a
Banach space E, there exists a bounded linear functional ⇤ P E^{˚} with }⇤} “ 1 and

⇤pxq “ }x}. For E “ L^{8}pXq, it follows that this ⇤ P E^{˚} cannot be represented as

⇤g “≥

Xxg,¨ydµfor anyg PL^{1}pXq. This is one way of seeing that the Riesz representation
theorem is false for p“ 8.

1

Problem Set 4

Problem 3

(a) Show that ifpM, dqis a metric space containing an uncountable subsetS ÄM such that every pair of distinct points x, y PS satisfies dpx, yq• ✏ for some fixed ✏° 0, thenM is not separable.

(b) SupposepX, µqcontains infinitely many disjoint subsets with positive measure. Show
thatL^{8}pXqcontains an uncountable subsetS ÄL^{8}pXq, consisting of functions that
take only the values 0 and 1, such that }f ´g}L^{8} “1 for any two distinct f, gPS.

Conclude thatL^{8}pXqis not separable.

Hint: If you’ve forgotten or never seen the proof via Cantor’s diagonal argument that R is uncountable, looking it up may help.

(c) Let LpHq denote the Banach space of bounded linear operators H Ñ H on a
separable Hilbert spaceHoverKP tR,Cu. Show that any orthonormal basistenu^{8}n“1

ofH gives rise to a continuous linear inclusion
:`^{8} ãÑLpHq,

where `^{8} denotes the Banach space of bounded sequences t n PKu^{8}n“1 with norm
}t nu}`^{8} :“sup_{n}_{PN}| n|, and pt nuq PLpHqis uniquely determined by the condi-
tion pt nuqej :“ jej for all jPN.

Comment: It is not hard to show that every subset of a separable metric space is also
separable. Since `^{8} “L^{8}pN,⌫q for the counting measure⌫, parts (b) and (c) thus imply
thatLpHqis not separable.

Problem 4 p˚q

This problem deals with weak convergence xn á x. Assume H is a separable Hilbert
space over K P tR,Cu with orthonormal basis tenu^{8}_{n“1}, and consider a sequence of the
form xn :“ nenPH for some nPK. Prove:

(a) xná0 whenever the sequence n is bounded. [3pts]

(b) If the sequence n is unbounded, then xn is not weakly convergent. [5pts]

Hint: Show that limnÑ8xej, xny “ 0 for every j P N and conclude that if xn á x
then x“0. Then associate to any subsequence with | nj|•j for j “ 1,2,3, . . . an
element of the form v “∞_{8}

j“1ajenj PH such thatxv, xnjy Û0 as jÑ 8.

Remark: We will later use a general result called the “uniform boundedness principle”

to show that weakly convergent sequences must always have bounded norms. But you should not use that result here, since we have not proved it.

(c) If | ^{n}|§?

n for all nPN, then every weakly open neighborhood of 0 PH contains infinitely many elements of the sequence xn. [5pts]

Comment: If the weak topology onHwere metrizable, then one could deduce from part (c) that a subsequence of ?

nen converges weakly to 0, contradicting part (b). It follows the- refore that the weak topology on an infinite-dimensional Hilbert space is not metrizable.

Problem 5

Find a sequence fn P L^{p}pRq for 1 † p † 8 that converges weakly to 0 but satisfies
}fn}^{L}^{p} “1 for alln, and deduce thatfn has noL^{p}-convergent subsequence.

2

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