Paper 2, Section II, 11F

Topics in Analysis | Part II, 2010

Let

Br(0)={(x,y)∈R2:x2+y2<r2},B_{r}(0)=\left\{(x, y) \in \mathbb{R}^{2}: x^{2}+y^{2}<r^{2}\right\},

B=B1(0)B=B_{1}(0), and

C={(x,y)∈R2:x2+y2=1}C=\left\{(x, y) \in \mathbb{R}^{2}: x^{2}+y^{2}=1\right\}

Let D=B∪CD=B \cup C.

(i) State the Brouwer fixed point theorem on the plane.

(ii) Show that the Brouwer fixed point theorem on the plane is equivalent to the nonexistence of a continuous map F:D→CF: D \rightarrow C such that F(p)=pF(p)=p for each p∈Cp \in C.

(iii) Let G:D→R2G: D \rightarrow \mathbb{R}^{2} be continuous, 0<ϵ<10<\epsilon<1 and suppose that

∣p−G(p)∣<ϵ|p-G(p)|<\epsilon

for each p∈Cp \in C. Using the Brouwer fixed point theorem or otherwise, prove that

B1−ϵ(0)⊆G(B)B_{1-\epsilon}(0) \subseteq G(B)

[Hint: argue by contradiction.]

(iv) Let q∈Bq \in B. Does there exist a continuous map H:D→R2\{q}H: D \rightarrow \mathbb{R}^{2} \backslash\{q\} such that H(p)=pH(p)=p for each p∈Cp \in C ? Justify your answer.

Typos? Please submit corrections to this page on GitHub.