What does the notation $f[E]$ mean with $f:A\rightarrow B$ and $E \subseteq A$

Question

Context:

Show or disprove $f[\cap\chi]=\cap\{f[X]:X\in\chi\}$ for all $\chi\subseteq\mathcal{P}(A)$ with $\chi\neq\emptyset$

I don't know how to start because I don't know what the cornered brackets mean ín settheory.

Would be nice if somebody could give me a hint how to approach this excercise.

Answer 1

f[A] = { f(x) : x in A }.
f[A] can also be written as f(A).
f[A] is the more precise notation.