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Re:What's the difference between 0/0 and 1/0? by codethief on Friday May 24, 2019 @08:07PM (#20006313)
The author's explanation is based on how the rationals are defined explicitly but this leaves the question open whether the rationals couldn't be extended somehow, so that the inverse of 0 is suddenly defined in this extension. More generally, couldn't there be other fields where 0 has a multiplicative inverse?

Of course the answer to both questions is negative but it's the consequence of a much more general fact: In any field the additive zero element can never have a multiplicate inverse. (If it did, the field's other algebraic operations would suddenly lead to contradictions.) And in this sense, there is also no difference between the expressions 1/0 = 1 · 0⁻¹ and 0/0 = 0 · 0⁻¹ because 0⁻¹ simply doesn't exist.

I've always found this explanation much more illuminating. No matter what you do, you can never have 0⁻¹!

Re:What's the difference between 0/0 and 1/0? by mjd on Friday May 24, 2019 @11:51PM (#20006994)
I did actually write a separate article on this exact subject, which I think is also pretty good.

https://math.stackexchange.com/a/125208/25554

Re:What's the difference between 0/0 and 1/0? by codethief on Saturday May 25, 2019 @04:00PM (#20011006)
Thanks for the link! I liked your explanation that

> [the] warped numbers are like a hole that you can fall into but you can't climb out of, and 0/0 is a deeper hole inside the first hole.

Re:What's the difference between 0/0 and 1/0? by kccqzy on Saturday May 25, 2019 @01:45AM (#20007335)
I've sometimes used in Alexandroff one-point compactification in scenarios where you really "need the zero to have an inverse" and it works well in certain applications.
Re:What's the difference between 0/0 and 1/0? by pstch on Friday May 24, 2019 @11:59PM (#20007019)
Although they aren't a field as such (if I'm remembering well, because they don't form a division algebra), sedenions (16-tuples, and also further applications of the Cayley-Dickson construction) do have zero divisors (84 of them).

This makes giving any kind of sense to 0^(-1) even harder for me (when 0 is a sedenion).

Re:What's the difference between 0/0 and 1/0? by chess93 on Saturday May 25, 2019 @10:34AM (#20009286)
No denominators for zero divisors in the localization of any ring!!! [English: No denominators where you can multiply the denominator by anything nonzero to get zero.]

Technically, many definitions of localization do allow for zero/zero divisors to be included (we can use any multiplicatively closed set) however by the definition of localization all of the elements in our localized ring become equal to each other.

The mathematical meaning of a/b = c/d in a general ring is that there exists some value s in the set of legal denominators where s(ad - c*b) = 0. If 0 is an allowable denominator then a/b = c/d for every a,b,c,d.

Re:What's the difference between 0/0 and 1/0? by daveFNbuck on Friday May 24, 2019 @10:24PM (#20006739)
You can have a multiplicative inverse of 0 if 0 = 1. This results in a field with only a single element.
Re:What's the difference between 0/0 and 1/0? by gizmo686 on Friday May 24, 2019 @10:33PM (#20006762)
Unless your definition of field include the non-triviality axiom that 0 != 1.
Re:What's the difference between 0/0 and 1/0? by throwawaymath on Saturday May 25, 2019 @01:46PM (#20010266)
The definition of a field explicitly requires the additive and multiplicative identities to be distinct elements. You can't induce a field structure on a singleton set. You can construct a trivial ring, but the trivial ring does not comprise a trivial field because it fails to be an integral domain.

To be fair this is mathematical pedantry when we're talking about trivial objects. But in principle it's meaningful because every field you can construct will lose critical properties if you allow one element to be both the additive and multiplicative identity. Unfortunately authors don't always make it clear that you need both existence and uniqueness when they state the field axioms.

Re:What's the difference between 0/0 and 1/0? by Certhas on Saturday May 25, 2019 @05:15AM (#20007911)
Not to be confused with the field with one element :P

https://en.wikipedia.org/wiki/Field_with_one_element

Re:What's the difference between 0/0 and 1/0? by sunstone on Friday May 24, 2019 @08:25PM (#20006373)
Yes this seems like the right answer, both results are undefined so they are the same but not equal.