A Hilbert space H is a pre-Hilbert space which is complete with respect to the norm induced by the inner product. nite dimensional normed space is complete. The example we had from the beginning of the course is l2 with the extension of (3.12) Completeness was shown earlier. 3. Orthonormal sets Proposition 20 (Bessel's inequality).
If V H is a subspace of a Hilbert space which contains a closed subspace of nite codimension in H { meaning V W where W is closed and there are nitely many elements ei 2 H; i = 1; : : : ; N such that every element u 2 H is of the form then V itself is closed. So, this takes care of the case that K = T has nite rank!
C and 'r : E u v ! C such that 'l u(v) = hu; vi; is continuous and linear, so that 'r 2 E0. To simplify notation, we write 'v instead of 'r v. Theorem ?? is generalized to Hilbert spaces as follows. Proposition 1.7. (Riesz representation theorem) Let E be a Hilbert space.
Theorem 12. Every separable pre-Hilbert space contains a maximal orthonor-mal set. Proof. Take a countable dense subset { which can be arranged as a sequence fvjg and the existence of which is the de nition of separability { and orthonormalize it.