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Demonstrating set properties of a map

  • #1
Hello, could you please help me regarding this question about a certain map (application).

Homework Statement


Demonstrate if A1 ⊂ A2 →ƒ(A1) ⊂ ƒ(A2)

2. Homework Equations

ƒ:A→B is a map
A1, A2⊂ A

The Attempt at a Solution


first assumed that (A1∪A2)⊆A
⇒ (ƒ(A1) ∪ ƒ(A2))⊆ ƒ(A)
then if A1 ⊂ A2
∴ ƒ(A1) ⊂ ƒ(A1)

I don't know if this demonstration is satisfying enough. I'm missing something related to properties of a function.
 

Answers and Replies

  • #2
I have also tried it this way,
I have assumed that
ƒ(A1⊂ A2) = ƒ(A1) ⊂ ƒ(A2)
∴A1⊂ A2→ƒ(A1⊂A2)
Thus, A1⊂ A2→ƒ(A1)⊂ƒ(A2)

This one looks better, doesn't it?
 
  • #3
Ray Vickson
Science Advisor
Homework Helper
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Hello, could you please help me regarding this question about a certain map (application).

Homework Statement


Demonstrate if A1 ⊂ A2 →ƒ(A1) ⊂ ƒ(A2)

2. Homework Equations

ƒ:A→B is a map
A1, A2⊂ A

The Attempt at a Solution


first assumed that (A1∪A2)⊆A
⇒ (ƒ(A1) ∪ ƒ(A2))⊆ ƒ(A)
then if A1 ⊂ A2
∴ ƒ(A1) ⊂ ƒ(A1)

I don't know if this demonstration is satisfying enough. I'm missing something related to properties of a function.
None of you "proofs" is anything of the kind (unless you are citing some results in your textbook or course notes that have already established the relationships you are using). As far as I can see you have just written down some relationships without any verification whatsoever.

Go back to the basics: (i) for a set ##A## and a function ##f##, what is the definition of ##f(A)?## (ii) for two sets ##A_1, A_2##, what is meant by the assertions ##A_1 \subset A_2## or ##f(A_1) \subset f(A_2)?## You need to look at individual elements ##f(x)## for ##x \in A.##
 

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