Preimage

From CS2800 wiki

If [math]f : A \href{/cs2800/wiki/index.php/%5Cto}{\to} B [/math], and if [math]y \href{/cs2800/wiki/index.php/%5Cin}{\in} B [/math], then the preimage [math]f^{-1}(y) [/math] of [math]y [/math] refers to the set of elements that map to [math]y [/math], i.e. [math]f^{-1}(y) := \{x \mid f(x) = y\} [/math].