Paper 1, Section II, G

Linear Analysis
Part II, 2011

State a version of the Stone-Weierstrass Theorem for real-valued functions on a compact metric space.

Suppose that K:[0,1]2RK:[0,1]^{2} \rightarrow \mathbb{R} is a continuous function. Show that K(x,y)K(x, y) may be uniformly approximated by functions of the form i=1nfi(x)gi(y)\sum_{i=1}^{n} f_{i}(x) g_{i}(y) with fi,gi:[0,1]Rf_{i}, g_{i}:[0,1] \rightarrow \mathbb{R} continuous.

Let X,YX, Y be Banach spaces and suppose that T:XYT: X \rightarrow Y is a bounded linear operator. What does it mean to say that TT is finite-rank? What does it mean to say that TT is compact? Give an example of a bounded linear operator from C[0,1]C[0,1] to itself which is not compact.

Suppose that (Tn)n=1\left(T_{n}\right)_{n=1}^{\infty} is a sequence of finite-rank operators and that TnTT_{n} \rightarrow T in the operator norm. Briefly explain why the TnT_{n} are compact. Show that TT is compact.

Hence, show that the integral operator T:C[0,1]C[0,1]T: C[0,1] \rightarrow C[0,1] defined by

Tf(x)=01f(y)K(x,y)dyT f(x)=\int_{0}^{1} f(y) K(x, y) d y

is compact.