Is it ensure, insure, or assure?
Relating to a mathematical operation whose nature or effect is the opposite of another operation. For example, addition and subtraction are inverse operations, as are multiplication and division.
Given a function, f : D -> C, a function g : C -> D is called a left inverse for f if for all d in D, g (f d) = d and a right inverse if, for all c in C, f (g c) = c and an inverse if both conditions hold. Only an injection has a left inverse, only a surjection has a right inverse and only a bijection has inverses. The inverse of f is often written as f with a -1 superscript.