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mliuzzolino
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Homework Statement
Let f : A → B be a function and let S, T [itex]\subseteq[/itex] A and U, V [itex]\subseteq[/itex] B:
Prove that if U [itex]\subseteq[/itex] V; then f-1(U) [itex]\subseteq[/itex] f-1(V):
Homework Equations
Preimage: f-1(U) = {x [itex]\in[/itex] f-1(U) [itex]\ni[/itex] f(x) [itex]\subseteq[/itex] U}
The Attempt at a Solution
Assume U [itex]\subseteq[/itex] V.
Let x [itex]\in[/itex] f-1(U), then f(x) = y [itex]\subseteq[/itex] U.
Since U [itex]\subseteq[/itex] V and y [itex]\in[/itex] U, then y [itex]\in[/itex] V.
Then by f-1(V), for y [itex]\in[/itex] V, x [itex]\subseteq[/itex] f-1(V).
Therefore, f-1(U) [itex]\subseteq[/itex] f-1(V).
Q.E.D.