(i) Let V be a Banach space and let L ⊆ V be a linear manifold. Show that L, the closure of L, is a subspace of V .
(ii) Let H be a Hilbert space. Let B be an orthonormal set in H. Show that span B is dense in H if and only if B is an orthonormal basis.
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Expert's answer
2021-02-24T07:04:00-0500
CORRECTED SOLUTION
1. Let L ⊆ V be a linear manifold, and Lˉ be its closure. For every x∈Lˉ there exist a sequence {xn}n=1∞⊂L which is converges to x.
Let a,b∈Lˉ are arbitrary vectors, a=limn→+∞an, b=limn→+∞bn with an,bn∈L for every n. Then an+bn∈L and limn→+∞(an+bn)=a+b. Hence, a+b∈Lˉ .
Let λ be any scalar constant, then λan∈L for every n and limn→+∞λan=λlimn→+∞an=λa.
Hence, λa∈Lˉ .
Since Lˉ is a closer of L, it is shown that Lˉ is a closed linear subspace of V.
2. if B is an orthonormal basis of H, then every element x in H may be approximated by a finite linear combinations of vectors from B, which are form span B. Hence, span B is dense.
Conversely, let span B is dense in H. Since B is an orthonormal set, for any x=y∈B∣x−y∣=2 . Since H is separable, this means that B must be countable or finite. Let B={v1,v2,...} and x be an element of H. We show that the series xn=∑k=1n⟨x,vk⟩vk converges to x.
Let ε>0 and ∣x−∑k=1nλkvk∣<ε (it is possible to find such linear combination, since span B is dense in H). Then ⟨x−xn,vk⟩=⟨x,vk⟩−⟨xn,vk⟩=⟨x,vk⟩−⟨x,vk⟩=0, hence ⟨x−xn,xn−∑k=1nλkvk⟩=0 and, by Pythagorean theorem, ∣x−∑k=1nλkvk∣2=∣(x−xn)+(xn−∑k=1nλkvk)∣2=∣x−xn∣2+∣xn−∑k=1nλkvk∣2 , which implies ∣x−xn∣2=∣x−∑k=1nλkvk∣2−∣xn−∑k=1nλkvk∣2<ε2 and ∣x−xn∣<ε.
For instance, if we take ∑k=1nλkvk=xn−1 , then we have ∣x−xn∣2=∣x−xn−1∣2−∣xn−xn−1∣2≤∣x−xn−1∣2 which implies ∣x−xn∣≤∣x−xn−1∣.
Therefore, the sequense ∣x−xn∣ is monotonous, non-negative and starting from some n is lesser than ε. This means xn converges to x and, therefore, B is an orthonormal basis in H.
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