Why does $A:X\to Y$ where $X$ is a banach space and $Y$ is a normed space, where $A$ is a surjective bounded linear operator, where:

$$\|x\|_X \leq C \|Ax\|_Y,\quad C\gt 0$$

Mean that $A$ is also an injective operator?

I can't see it. Not sure where to start, it doesn't seem like it immediately jumps out at me.


If $Ax = 0$, then $0\le \|x\|_X\le C\|Ax\|_Y = 0$, so .....

  • $\begingroup$ If $Ax=0$ then $x=0$, and hence the kernel is trivial and the operator is injective right $\endgroup$ – Functional Analysis Oct 24 '15 at 8:35
  • $\begingroup$ Yes, you are right.@FunctionalAnalysis $\endgroup$ – user99914 Oct 24 '15 at 8:36
  • $\begingroup$ And it being injective implies it has an inverse operator? $\endgroup$ – Functional Analysis Oct 24 '15 at 8:41
  • $\begingroup$ No in general, it has to be onto.@FunctionalAnalysis $\endgroup$ – user99914 Oct 24 '15 at 9:01

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