I Proof about pre-images of functions

AI Thread Summary
The discussion revolves around proving two properties related to functions and their pre-images: that L is a subset of f-inverse of f(L), and that f of f-inverse of P is a subset of P. Participants express difficulty in finding a proof and seek guidance. A hint suggests tracking a point from L through the function and its inverse to clarify the relationships. The conversation emphasizes the importance of logical order in the proof steps. Ultimately, the focus remains on understanding the definitions and applying them correctly to complete the proofs.
PhysicsRock
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The problem reads: ##f:M \rightarrow N##, and ##L \subseteq M## and ##P \subseteq N##. Then prove that ##L \subseteq f^{-1}(f(L))## and ##f(f^{-1}(P)) \subseteq P##.
My co-students and I can't find a way to prove this. I hope, someone here will be able to help us out. It would be very appreciated.

Thank you in advance and have a great day everyone.
 
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If this is a textbook homework type of problem, then there is a section and a format for that and we are only allowed to give hints and guidance.
Hint: pick a point in the smaller subset side and track it through the operations.
 
FactChecker said:
If this is a textbook homework type of problem, then there is a section and a format for that and we are only allowed to give hints and guidance.
I guess I figured it out anyway, at least I tried. Thank you for the advice. I'll ask for a specific hint etc. next time.
 
PhysicsRock said:
The problem reads: ##f:M \rightarrow N##, and ##L \subseteq M## and ##P \subseteq N##. Then prove that ##L \subseteq f^{-1}(f(L))## and ##f(f^{-1}(P)) \subseteq P##.
My co-students and I can't find a way to prove this. I hope, someone here will be able to help us out. It would be very appreciated.

Thank you in advance and have a great day everyone.
Let ##x \in L##. Then ##y = f(x) \in f(L)##. Now, what is, by definition, ##f^{-1}(f(L))##? And why is ##x \in f^{-1}(f(L))##?

Hint: it might help conceptually (be less confusing) to let ##X = f(L)## so that ##y = f(x) \in X## and show that ##x \in f^{-1}(X)##.

PS the trick with these proofs is to get all the logical steps in the right order.
 
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