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fresh_42

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**1.**(solved by @nuuskur ) Let ##K## be a non-empty compact subset of ##\Bbb{C}##. Construct a bounded operator ##u: H \to H## on some Hilbert space ##H## that has spectrum ##\sigma(u) =K##. (MQ)

**2.**Let ##f,g:[0,2]\to\mathbb{R}## be continuous functions such that ##f(0)=g(0)=0## and ##f(2)=g(2)=2##. Show that there exist ##a,b\in [0,2)\,,a\neq b\,.## such that ##f(a)-f(b)## and ##g(a)-g(b)## are both integers. (IR)

**3.**(solved by @nuuskur ) Let ##V## be a normed vector space and ##W## be a closed linear subspace. If ##V/W## is a Banach space for the quotient norm, show that ##V## is a Banach space as well. (MQ)

**4.**(solved by @nuuskur ) Let ##(\Omega, \mathcal{F}, \Bbb{P})## be a probability space and ##X: \Omega \to \Bbb{R}## be a random variable. If ##\Bbb{P}(X \in A) \in \{0,1\}## for all Borel sets ##A##, show that there is a constant ##c \in \Bbb{R}## such that ##\Bbb{P}(X= c) = 1## (i.e. ##X## is constant almost surely). (MQ)

**5.**Let ##X,Y## be Banach spaces over the field ##\mathbb{R}## or ##\mathbb{C}##. By ##B_X(\hat x,R)## we denote the open ball of this space:

$$B_X(\hat x,R)=\{x\in X\mid \|x-\hat x\|_X<R\},$$ and ## \overline B_X(\hat x,R)=\{x\in X\mid \|x-\hat x\|_X\le R\}.##

Consider a continuous bilinear function ##A:X\times X\to Y## such that

$$\|A(x_1,x_2)\|_Y\le a\|x_1\|_X\|x_2\|_X,\quad A(x_1,x_2)=A(x_2,x_1).$$

Let ##B:X\to Y## stand for a bounded linear operator onto:

$$\overline B_Y(0,b)\subseteq B(\overline B_X(0,1))$$ with some positive constant ##b##. Let ##C## stand for a fixed element of ##Y,\quad \|C\|_Y=c##.

Show that

If ##D=b^2-4ac\ge 0## then the equation

$$A(x,x)+Bx+C=0$$ has a solution. (WR)

**6.**(solved by @nuuskur ) Show that in

**ZF**the Tychonoff theorem (product of compact spaces is compact) implies the axiom of choice. (MQ)

**7.**(solved by @nuuskur ) Let ##f: (X,d_X) \to (Y,d_Y)## be a map between metric spaces (not necessarily continuous!). Show that ##D(f):= \{x \in X\mid f \mathrm{\ is \ not \ continuous \ at \ x}\}## is a Borel-measurable set of ##X##. (MQ)

**8.**(solved by @benorin , @Fred Wright , and @nuuskur ) Let ##F## be a meromorphic function (holomorphic up to isolated poles) in ##\mathbb{C}## with the following properties:

(1) ##F## is holomorphic (complex differentiable) in the half plane ##H(0)=\{z\in \mathbb{C}\, : \,\Re(z)>0\}.##

(2) ##zF(z)=F(z+1).##

(3) ##F## is bounded in the strip ##\{z\in \mathbb{C}\, : \,1\leq\Re(z)\leq 2\}.##

Show that ##F(z)=F(1)\Gamma(z).## (FR)

**9.**(solved by @PeroK ) Show that there exists no continuous function ##g: \Bbb{C}\setminus \{0\}\to \Bbb{C}## such that ##e^{g(z)} = z## for all ##z\in \Bbb{C}\setminus \{0\}## (i.e. there is no continuous logarithm on ##\Bbb{C}\setminus \{0\})##. (MQ)

(

**Corrected version**, credits go to @PeroK for pointing out and solving the original problem.)

**10.**(solved by @PeroK ) Show that if ##f## is any continuous real function and ##n## any positive number (FR),

$$

I:=\int_{n^{-1}}^{n} f\left(x+\dfrac{1}{x}\right)\,\dfrac{\log x}{x}\,dx =0.

$$

**High Schoolers only**

11.(solved by @Not anonymous ) Let ##a<b<c<d## be real numbers. Sort ##x=ab+cd\, , \,y=bc+ad\, , \,z=ac+bd## and prove it.

11.

**12.**(solved by @Not anonymous ) Prove ##\overline{CP}^2=\overline{AP}\cdot \overline{BP}\,.##

**13.**How big is the probability for two pocket aces in Texas Hold'em? Assume we have seen a show down in a heads-up. How many possible combinations are there, how many combinations of possible starting hands can the opponents have? How many possible community cards?

**14.**(solved by @etotheipi ) Everybody knows that Schrödinger's cat is trapped in the box since ##1935.## Not well known is the fact, that the radioactive material was ten ##{}^{14}C## isotopes. Calculate the probability that the cat is still alive.

**15.**(solved by @ItsukaKitto ) Show that there is no rational solution for ##p^2+q^2+r^2=7.##

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