MHB Why is the following not true: f(f^(-1)(B))=B?

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The title says it: why is the following not true: $f(f^{-1}(B))=B$?

Thanks!
 
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Re: Why is the following not true: $f(f^{-1}(B))=B$?

OhMyMarkov said:
The title says it: why is the following not true: $f(f^{-1}(B))=B$?

Thanks!
You have not given a context for this, but in general if you have a function $f:A\to B$ there is no reason to suppose that the range of $f$ is the whole of $B$. If $f$ is not surjective then $f(f^{-1}(B)) = f(A) \ne B$.
 

Thread 'Apparent counter-example to Cauchy-Goursat theorem (Complex Analysis)'

I posted this question on math-stackexchange but apparently I asked something stupid and I was downvoted. I still don't have an answer to my question so I hope someone in here can help me or at least explain me why I am asking something stupid. I started studying Complex Analysis and came upon the following theorem which is a direct consequence of the Cauchy-Goursat theorem: Let ##f:D\to\mathbb{C}## be an anlytic function over a simply connected region ##D##. If ##a## and ##z## are part of...
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