Weak Limit in Normed Vector Space is Unique

Theorem
Let $\struct {X, \norm \cdot}$ be a normed vector space.

Let $x, y \in X$.

Let $\sequence {x_n}_{n \mathop \in \N}$ be a sequence in $X$ such that:


 * $x_n \rightharpoonup x$

and:


 * $x_n \rightharpoonup y$

where $\rightharpoonup$ denotes weak convergence.

Then:


 * $x = y$

Proof
Let $\struct {X^\ast, \norm \cdot_{X^\ast} }$ be the normed dual space of $\struct {X, \norm \cdot}$.

Since:


 * $x_n \rightharpoonup x$

we have:


 * $\map f {x_n} \to \map f x$ for each $f \in X^\ast$.

Since:


 * $x_n \rightharpoonup x$

we have:


 * $\map f {x_n} \to \map f y$ for each $f \in X^\ast$.

From Convergent Sequence in Normed Vector Space has Unique Limit, we have:


 * $\map f x = \map f y$ for each $f \in X^\ast$.

From Normed Dual Space Separates Points, we therefore have:


 * $x = y$