A3.3 B3.2

Functional Analysis | Part II, 2003

(i) Let pp be a point of the compact interval I=[a,b]⊂RI=[a, b] \subset \mathbb{R} and let δp:C(I)→R\delta_{p}: C(I) \rightarrow \mathbb{R} be defined by δp(f)=f(p)\delta_{p}(f)=f(p). Show that

δp:(C(I),∥⋅∥∞)→R\delta_{p}:\left(C(I),\|\cdot\|_{\infty}\right) \rightarrow \mathbb{R}

is a continuous, linear map but that

δp:(C(I),∥⋅∥1)→R\delta_{p}:\left(C(I),\|\cdot\|_{1}\right) \rightarrow \mathbb{R}

is not continuous.

(ii) Consider the space C(n)(I)C^{(n)}(I) of nn-times continuously differentiable functions on the interval II. Write

∥f∥∞(n)=∑k=0n∥f(k)∥∞ and ∥f∥1(n)=∑r=0n∥f(k)∥1\|f\|_{\infty}^{(n)}=\sum_{k=0}^{n}\left\|f^{(k)}\right\|_{\infty} \quad \text { and } \quad\|f\|_{1}^{(n)}=\sum_{r=0}^{n}\left\|f^{(k)}\right\|_{1}

for f∈C(n)(I)f \in C^{(n)}(I). Show that (C(n)(I),∥′⋅∥∞(n))\left(C^{(n)}(I),\left\|^{\prime} \cdot\right\|_{\infty}^{(n)}\right) is a complete normed space. Is the space (C(n)(I),∥⋅∥1(n))\left(C^{(n)}(I),\|\cdot\|_{1}^{(n)}\right) also complete?

Let f:I→If: I \rightarrow I be an nn-times continuously differentiable map and define

μf:C(n)(I)→C(n)(I) by g↦g∘f.\mu_{f}: C^{(n)}(I) \rightarrow C^{(n)}(I) \quad \text { by } \quad g \mapsto g \circ f .

Show that μf\mu_{f} is a continuous linear map when C(n)(I)C^{(n)}(I) is equipped with the norm ∥⋅∥∞(n)\|\cdot\|_{\infty}^{(n)}.

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