Consider finite-dimensional vector spaces, on which two norms ‖‖1 and ‖‖2 are always equivalent. Then a sequence of vectors vk→v w.r.t. norm 1 iff vk→v w.r.t. norm 2.
The question is about sequence vk‖vk‖1→v‖v‖1 w.r.t. norm 1, will this limit imply vk‖vk‖2→v‖v‖2 w.r.t. norm 2 (or maybe a weaker claim that one convergence implies the other but the limit might be different)? I have trouble showing this is true. If this is false, anyone can help provide a counterexample? Thanks!
For example, (k,√k),k=1,2,... satisfies (k,√k)‖(k,√k)‖p=(k,√k)(kp+kp2)1p→(1,0) for any p-norm.
Answer
Supposedly v≠0. Let uk=vk/‖vk‖1 and u=v/‖v‖1≠0. By assumption, uk→u (with respect to both norms, because all norms are equivalent) and hence ‖uk‖2→‖u‖2 (because the norm function is continuous). Therefore
‖uk‖uk‖2−u‖u‖2‖2≤1‖uk‖2‖uk−u‖2+|1‖uk‖2−1‖u‖2|‖u‖2→0.
Consequently, vk/‖vk‖2=uk/‖uk‖2→u/‖u‖2=v/‖v‖2.
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