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Weak Compactness and Closedness of Bounded Subsets of the Small Space of a Hilbert Triple

In a Hilbert triple (H,V,A), a V-bounded sequence has a subsequence converging weakly in V and in H to a point of V with the same norm bound; a V-bounded sequence converging in H has its limit in V with |x|_V <= R.

Statement

Let R\mathbb{R} be the ordered field of real numbers, with the notation of that item, let N\mathbb{N} be the set of natural numbers, and let (H,V,A)(H,V,A) be a Hilbert triple, with the inner products, norms and distances of HH and VV written with the subscripts HH and VV, such that (V,dV)(V,d_{V}) is separable. Let C∈RC\in\mathbb{R} and let (xm)m∈N(x_{m})_{m\in\mathbb{N}} be a sequence in VV with ∣xm∣V≤C|x_{m}|_{V}\le C for every m∈Nm\in\mathbb{N}. Convergence and weak convergence in HH or in VV refer to the respective distance and inner product. Then the following hold; since CC and (xm)m∈N(x_{m})_{m\in\mathbb{N}} are arbitrary, each claim holds for every real CC and every sequence in VV bounded by CC in ∣⋅∣V|\cdot|_{V}.

1. (Weakly convergent subsequence) There exist a subsequence (xnj)j∈N(x_{n_{j}})_{j\in\mathbb{N}} of (xm)m∈N(x_{m})_{m\in\mathbb{N}} and a point w∈Vw\in V with ∣w∣V≤C|w|_{V}\le C such that (xnj)j∈N(x_{n_{j}})_{j\in\mathbb{N}} converges weakly to ww in VV and converges weakly to ww in HH.

2. (Closure) If (xm)m∈N(x_{m})_{m\in\mathbb{N}} converges to a point x∈Hx\in H in (H,dH)(H,d_{H}), then x∈Vx\in V, ∣x∣V≤C|x|_{V}\le C, and (xm)m∈N(x_{m})_{m\in\mathbb{N}} converges weakly to xx in VV.

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