5 ms·

> Instead of writing Xi, they should really write X[i] In math, x_i is i-th element in a set/list and X[i] is the ring of polynomials.

• Someone 11y ago
  No, both are whatever you want them to be. http://mathworld.wolfram.com/SquareBracket.html http://mathworld.wolfram.com/SquareBracket.html gives some options for square brackets. https://en.m.wikipedia.org/wiki/Bracket_(mathematics) https://en.m.wikipedia.org/wiki/Bracket_(mathematics) has quite a few more (do not forget to check out the 'See also' section) And x_i could for example mean 'x in base i' (http://mathforum.org/library/drmath/view/57226.html http://mathforum.org/library/drmath/view/57226.html) Mathematicians invent notation to suit them in whatever problem they are working on. That notation doesn't have to be globally consistent.
  • chelovek 11y ago
    That's true. These just so happen to be ubiquitous. We can agree to call 2 a unit. To most, though, it won't be.
    • gizmo686 11y ago
      Nitpick, under standard definitions, 2 is a unit, over the field of rationals (or reals, or many other systems). EDIT: for a less trivial example, 2 is a unit in the integers mod 9, but 3 isn't.