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if T :X -> Y nls's is a linear transformation, we define the norm of T, ||T||: sup{||T|| : ||x||<=1}

I want to show that for X not = {0}

||T||: sup{||T|| : ||x|| = 1} frustratingly the books all assume that this step is obvious... I don't see how.

Intuitively I can see that is true, using the fact that (I think) T is a bounded function, and we can manipulate things to make the whole function be contained within a closed unit sphere...

Have always had a mental block with inf's and sups...don't know why...