4
$\begingroup$

I've tried googling this usage and understanding the results but I'm struggling to make intuitive sense of it. So my question is, what is the phrase "up to" understood to mean, and what are some prominent examples of this?

$\endgroup$
2
  • 1
    $\begingroup$ "Finding all objects up to homeomorphism/isotopy/isomorphism/conjugacy" means "describe a list of examples so that any other example of this type is homeomorphic/isotopic/isomorphic/conjugate to one of the known examples". For instance, the groups of order 4 "up to isomorphism" are ${\mathbf Z}/(4)$ and ${\mathbf Z}/(2) \times {\mathbf Z}/(2)$. That means any group of order 4 is isomorphic to one of those (and, actually, not to both; the examples are not isomorphic to each other). $\endgroup$
    – KCd
    Apr 2, 2013 at 3:08
  • $\begingroup$ If I open a silverware drawer, and all of the forks are of the same size and material, I might analogously say "all of these forks are the same (up to the designs put on the handle)". All of the forks are the same as far as dining ware usage is concerned, and they differ only superficially. $\endgroup$ May 1, 2020 at 17:18

2 Answers 2

4
$\begingroup$

"X up to Y" means that X is an element of some set (or possibly proper class) but, instead of talking about elements of the set, we want to talk about equivalence classes of elements under some equivalence relation given by Y. Equivalence relations are ubiquitous in mathematics, hence so is the phrase "up to." For example, "classify compact surfaces up to homeomorphism" means we want to classify equivalence classes of $2$-dimensional compact surfaces under the relation "$S_1$ is homeomorphic to $S_2$."

$\endgroup$
1
$\begingroup$

You may be familiar with the integers modulo 3. This is a field with three elements. In fact, it is the only field with three elements, up to isomorphism.

But what are the integers modulo 3?

One might say it is the set whose three elements are

$$ 3\mathbb{Z} \qquad \qquad 1 + 3\mathbb{Z} \qquad \qquad 2 + 3\mathbb{Z}$$

(i.e the three equivalence classes of $\mathbb{Z}$ modulo the relation $x \equiv y \bmod 3$), with addition and multiplication defined the usual way.

However, one might say it is the set whose three elements are

$$ 0 \qquad \qquad 1 \qquad \qquad 2 $$

and addition and multiplication are defined by

$$ a \oplus b = \begin{cases} a + b & a + b < 3 \\ a + b - 3 & a + b \geq 3 \end{cases} $$

$$ a \odot b = \begin{cases} ab & ab < 3 \\ ab - 3 & ab \geq 3 \end{cases} $$

But wait there's more! One might instead say it is the set whose three elements are $\{ 0, 1, -1\}$, or occasionally one might even use $\{ 1, 2, 3 \}$.


The point is, while technically all of these give different fields, the difference between them is superficial. Except in odd circumstances, there is absolutely nothing to be gained by keeping them all different in your mind; instead, they should all be regarded as the same thing.

This example is extra nice, because not only are they all isomorphic fields, but they are uniquely isomorphic -- there is only one way to choose an isomorphism between any two of them.

$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.