Wikipedia:Reference desk/Archives/Mathematics/2021 September 11

= September 11 =

Motivation for studying mathematical structures

 * I've factored this and the following into separate subsections; it is IMO not appropriate to hijack another contributor's thread for extensive discussions on only tangentially related topics. --Lambiam 19:15, 11 September 2021 (UTC)

For my personal benefit, could you [JD] explain the motivation behind your question? I studied groups (and rings and fields) last year, and will study them further starting next month, but although I get reasonably high marks on my assignments by mechanically following the rules to answer questions (a state of affairs that Richard Feynman would not have liked), my appreciation of the topic is hindered by not having the slightest clue what groups are for, what good they do anyone, or what makes them interesting. Card Zero (talk) 11:37, 11 September 2021 (UTC)
 * It would be awkward to describe Noether's theorem without recourse to the concepts of group theory; the very first sentence of the seminal paper appeals to the concept of a Lie group, a mathematical rendering of the intuitive idea of continuous symmetry. --Lambiam 11:58, 11 September 2021 (UTC)
 * So, not the kind of symmmetry group relevant to Mattress, since those symmetries are discrete, if that's the right word for the opposite of continuous. (This is confusing, since a mattress is something you Lie on, but that may help it stick in my mind.) Thank you, this is a starting point, although now I'm just thinking about how utterly pointless the infinite identical symmetries of a sphere are. Card Zero  (talk) 12:39, 11 September 2021 (UTC)
 * Note that the surname of Sophus Lie is pronounced . --Lambiam 15:33, 11 September 2021 (UTC)
 * The conventional way of defining a sphere as a set of points gives no insight into its structure; a pointless understanding, as offered by $O(3) $\sim$ 2 × SO(3)$, is more enlightening. --Lambiam 15:45, 11 September 2021 (UTC)

Mathematical senses of tilde
Is our article Tilde well-written, do you think, and are you saying "approximately equals" with this particular twiddle, or something else? I have a faint memory that it can also be used like a variable for relations, so it would just mean "relates to (in some way not yet defined)". Card Zero (talk) 17:27, 11 September 2021 (UTC)
 * I used $~$ in the sense of "is equivalent to". More specifically, $O(3)$ is isomorphic to a disjoint union of two copies of $SO(3)$, abbreviated above as $2 × SO(3)$. This is initially merely a set isomorphism, but can be made into a full group isomorphism. If I can help it, I use $≈$ or $≃$ for "approximately equal", not a single tilde. I'd never use $~=$, since it can be read as "not equal". --Lambiam 19:15, 11 September 2021 (UTC)
 * I've seen ~ used to mean a generic relation whose properties you want to talk about, but which you don't want to define explicitly. One such use is in the article Equivalence relation. You'd probably only use it in reference to properties that are "equivalence like", since ~ is so often associated with isomorphism and other "equivalences". For an "order like" relation, if the ≤ symbol wasn't available then you'd probably use some variation such as ≲. See Help:Displaying a formula for more symbols similar to ~, the TeX markup for ~ itself is \sim, which produces $$\sim$$. AFAIK it's the same symbol to mean "approximately equal" and "is equivalent to"; such overloading of symbols is common and you have to rely on context to figure out the meaning in any specific instance, just as you have to use context to figure out the meaning of "bear" when you see it. --RDBury (talk) 23:40, 11 September 2021 (UTC)