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What are the easiest examples of a pairs of Banach spaces $X,Y$ such that

  • $X\subseteq Y$ ($X$ is a closed linear subspace of $Y$)
  • there is a bounded linear map $T\colon X\to Y$;
  • there is no bounded extension $\hat{T}\colon Y\to Y$ of $T$?

Needless to say, I am interested in the structure of the operator $T$ rather than in its existence.

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