Added a link to weakly reductive semigroup.

]]>.

]]>Weak reductive isn’t the same as cancellative, although the latter implies the former. What makes the two different is placement of quantifiers.

Note that in this case it doesn’t even make sense for the multiplicative semigroup to be cancellative, as 0 is never going to be cancelled. Instead, we can demand that there exists an element which can be cancelled from one of the sides.

Adam

]]>I added the word ’cancellative’.

]]>I am more familiar with ’cancellative’ than I am with ’reductive’. See cancellative monoid.

]]>In this article there is a remark which says that for unital rings commutativity of addition is not a needed axiom, but it’s necessary for nonunital rings.

I completed this remark with the following.

I’ve added a case of nonunital rings in which this axiom is not needed. The proof is pretty much the same as for the unital rings. Those are nonunital rings of which multiplicative semigroup is left/right weakly reductive i. e. $x\cdot a = x\cdot b$ for all $x$ implies $a = b$, or the same from the other side.

I wrote a brief (and probably a bit messy) article here on nlab about such semigroups if you need more information.

The only purpose of unity here (here meaning - this remark) is so that the multiplication can distinguish between the elements of our multiplicative semigroup. Of course this is the most basic property one would desire. Another property like this is left/right weak reduction.

Perhaps someone thinks that I should’ve written the weak reductive property more explicitly in the article, but I didn’t want to digress any more from the subject which are rings.

Adam

]]>Linked to category CRing.

]]>You should put *something* around that paragraph, wrapping it, something that allows to discern it as a new idea within the text that surropunds it. At least maybe a remark-environment.

I knew it had to be there, but even so I only found it after hitting *see changes*.

I added the following standard observation

The structure of an $A\otimes A^{op}$-ring $(R,\mu_R,\eta)$ is determined by the structure of $A$ as a ring, together with the two natural homomorphisms of rings $s = \eta(-\otimes 1_A):A\to R$ and $t=\eta(1_A\otimes -):A^{op}\to R$ which have commuting images ($s(a)t(a')=t(a')s(a)$, for all $a,a'\in A$).

This is very interesting when dualizing the notion of groupoid (algebra of functions/space duality) – source and target map in algebraic language get sometimes conveniently packed into $A\otimes A^{op}$-ring language, as in the case of bialgebroids.

]]>Started at *ring* an *Examples*-section. Just some very basic examples so far

There is an Stackexchange question here, with another historical reference.

]]>By the way, I had put some text into the *Idea-section*. Not meant to be perfect. Please edit as you see the need.

A google search did turn up this,

Yes, that’s already linked to in the entry.

but that does not seem to answer the question.

I believe I just answered the question in #3. :-)

]]>A google search did turn up this, but that does not seem to answer the question.

]]>I don’t know, I just meant to say that I haven’t seen any original text on Dedekind’s “Ordnungen”. Maybe he didn’t even write it up. He is just being credited for the idea (by Hilbert).

I have made further notes at *ring - References - History*

What is the *Dedekind’s original text* ?

Do you have a source for that?

Hm. Let’s see. That’s what they told me when I was a gullible student. I never checked the originals. The entry Mathworld – Ring tells its readers the same story:

The term was introduced by Hilbert to describe rings like […] By successively multiplying the new element […], it eventually loops around to become something already generated, something like a ring,

but apparently it’s just a story, not a review of Hilbert’s way of introducing the term.

Okay, so I went to Google books and read Hilbert’s original article

David Hilbert,

Die Theorie der algebraischen Zahlkörper, Jahresbericht der Deutschen Mathematiker-Vereinigung 4 (1879)

and there, in section 9.31, indeed no motivation like this is given. Instead it just says:

[…] ein Zahlring, Ring oder Integritätsbereich genannt

with a footnote that reads

Nach Dedekind “eine Ordnung”.

And that’s it. But so that means already that I was wrong, since neither Dedekind nor Hilbert meant to invoke the picture of clock arithmetic. (And Hilbert does not even mention anything as simple as $\mathbb{Z}_n$).

Now I don’t have Dedekind’s original text. Because also “Ordnung” is ambiguous. One meaning is “order” as in “ordered set”. But it also is used in the sense of “a collection of beings of the same nature” in the sense used as a taxonomic rank.

Dedekind can’t have meant “ordered set”. So he must have meant “taxonomical order”. Maybe thinking of a “taxonomy of numbers”? I don’t don’t know.

But anyway, I suppose that Hilbert’s “ring” is therefore also to be read as meaning “collection of beings”, as in “drug-dealer ring”. :-)

Which, I must say, is too bad.

]]>to explain the origin of the word “ring”.

I didn’t know that! Do you have a source for that?

]]>at the beginning of *ring* I have spelled out a more explicit definition. Also added the examples of rings on cyclic groups to explain the origin of the word “ring”.