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Let $$f \colon \mathbb{R} \rightarrow \mathbb{R}$$ be defined by $$f(x) = x^2$$ for all $$x$$ in $$\mathbb{R}$$. Let $$A = \{4, 3, 0, -1\}$$. Then
The pre-image $$f^{-1}(T) = \{x \mid f(x) \in T\}$$ is defined for every subset $$T$$, even when $$f$$ has no inverse function. Here $$x^2 \in \{4, 3, 0, -1\}$$ gives $$x = \pm 2$$, $$\pm\sqrt{3}$$ or 0, while $$x^2 = -1$$ has no real solution and simply adds nothing. So $$f^{-1}(A) = \{-2, 2, \sqrt{3}, -\sqrt{3}, 0\}$$.
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