Friday 19 September 2014

functional analysis - Show that $infty$-norm and $C^1$-norm are not equivalent.

Show that $\infty$-norm and $C^1$-norm are not equivalent.



For the $C^1([a,b],\mathbb{R})$ space, show that



$\displaystyle ||g||_\infty=sup_{a\leq t\leq b}|g(t)|$



and




$\displaystyle ||g||_{C^1}=sup_{a\leq t\leq b}|g(t)|+sup_{a\leq t\leq b}|g'(t)|$



are not equivalent.



My attempt:



$\displaystyle |g'(t)|>0 \implies sup_{a\leq t\leq b}|g'(t)|>0$



So then




$\displaystyle ||g||_\infty=sup_{a\leq t\leq b}|g(t)| \leq sup_{a\leq t\leq b}|g(t)|+sup_{a\leq t\leq b}|g'(t)| \leq p ||g||_{C^1}$ for constant $p \geq 1$.



If the two norms are not equivalent then I'm assuming that



$||g||_\infty \ngeq q ||g||_{C^1}$ for any constant $0

Is this a good approach or does anyone have a better idea?

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