Let $u \in C^0([0,T], H^{s-1}(\Bbb R^n)). $ Let $\{t_n \} \subset [0,T]$ such that $\lim_{n \to \infty} t_n = t_0$. Let $ u(t_n ) \to u(t_0) $ in the Sobolev space $H^{s-1} ( \Bbb R^n )$ for $s = 1,2, \cdots$. Assume $ \| u(t_n ) \|_{H^{s} (\Bbb R^n)} \leqslant R < \infty $ for all $n$.
Then how can we find a subsequence $\{t_k \}$ of $\{ t_n \}$ such that $u(t_k ) \to u(t_0) \;weakly$ in $H^s$ ?