Groups and Composition

For Translations:
  1. There exists an inverse mapping for each function
  2. There exists an identity mapping

These properties might seem trivial at first glance, but they are actually very important, because when these conditions are shown for any class of functions it can be proven that such a class is closed under composition (i.e. any series of translations can be composed to a single translation). In mathematical parlance this the same as saying that translations form an algebraic group.
Lecture 10 Slide 4 6.837 Fall '98