DaRT/DoME

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Jun 13 01:20
@rschwieb yeah I also think stacks is a good reference. But what I meant was that it doesn't treat this topic systematically and the results are a bit scattered.
Jun 12 15:40
I know that's not really a satisfying reference...
Jun 12 15:38
if you look up the page in stacks on a ring propety, you will often find the corresponding meta-proprerty if it has it. For example, looking up normal rings, we find this:

https://stacks.math.columbia.edu/tag/030B
Jun 12 15:35
Here's a start:
https://stacks.math.columbia.edu/tag/00EN
Jun 12 15:35
But the actual results when a property has this meta-property is of course spelled out in several places, for example in the stacks project
Jun 12 15:35
@rschwieb unfortunately, although this is used very much in algebraic geometry, I don't know a reference that really talks about this on a meta-level. I could only offer some German handwritten lecture notes (if I can find them!). I'll look into it
Jun 12 14:27
thinking about it, more generally "may be checked on a partition of unity" should also imply stable under finite products
Jun 12 14:25
@rschwieb there's even another "metametaproperty": any property that may be checked on stalks is stable under finite products :)
Jun 12 12:11
maybe these metaproperties are not so useful for automatic deduction, but it would be still nice to document which properties have these metaproperties
Jun 12 12:10
if you have further questions about this, feel free to ask. The names I suggested are not completely standard I think, but would clearly convey the concept to people trained in algebraic geometry. The concepts themselves, however, are very prevalent in any treatment of commutative algebra that is used as a basis for scheme theory
Jun 12 12:08
There's an interesting relation between these metaproperties (is this a metametaproperty?) which says that any property that may be checked on a partition of unity may also be checked on stalks, but the converse does not a hold. Interestingly, the property of being Noetherian can be checked on a partition of unity, but cannot be checked on stalks.

These two metaproperties are absolutely fundamental and natural in algebraic geometry.

There are also corresponding metaproperties for module-properties over commutative rings. For example, it is true that a module $M$ over $R$ is flat if and on
Jun 12 12:02
@rschwieb I would like to suggest two metaproperties for properties of (commutative) rings that are used frequently in commutative algebra in algebraic geometry. Both are about relating a ring $R$ having a property $P$ to certain localisations of $R$ having $P$.

(I think these properties are very unusual in the noncommutative setting though.)

The first one could be called "may be checked on stalks" (if you want a geometric name) or "may be checked at prime ideals" (for a more algebraic name). A commutative ring property $P$ has this metaproperty if for any commutative ring $R$, $R$ has $P
May 25 17:09
Unfreeze
May 6 12:49
the LotR reference is very apt
May 6 12:47
Thanks! I send you a message on discord btw
May 5 21:33
yeah I'm on discord
May 5 16:54
it's a bit odd that the abstract is as technical as it is and doesn't mention the amazing corollary with no size limitations...
May 5 16:52
and yes a Kronecker module is acutually a module over Γ_2(R)
May 5 16:51
I can mail you the paper, if you want
May 5 16:49
May 5 16:49
I can confirm that there are no size limitations in the final result
May 3 02:30
@rschwieb yeah $R^2=R \times R$
Apr 23 21:12
@rschwieb I suggested a ring with a curious property regarding endomorphism rings of modules over it
Jan 22 15:24
you have some stuff in the stacks project, and in some algebraic geometry books (e.g. EGA and Liu), but it's rarely covered
Jan 22 15:24
(and all the related stuff)
Jan 22 15:23
unfortunately, a lot of commutative algebra books leave out the topic of excellence
Jan 22 15:23
@rschwieb yes, it's a PID, hence regular so the regular locus is the entire spectrum and thus it is J-0
Jan 5 23:32
thus the example is not J_2
Jan 5 23:32
But excellent = universally catenary + G-ring + J_2
 

 Mathematics

Associated with Math.SE; for both general discussion & math qu...
May 29 20:28
seems correct
May 29 20:28
@Jakobian yeah I was thinking you could do something like this, but I forgot the little descriptive set theory I knew
May 29 20:22
a pointwise limit of Borel measurable functions is Borel measure and continous functions are Borel measure. Thus the derivative of a differentiable function is going to be Borel measure. So both f'(x) and lim f_n'(x) are Borel measure and hence the set on which they agree is Boreal measure
May 29 20:21
@leslietownes well, for starters, this set is going to be Borel measurable
May 5 19:50
no wait, $B$ might not be flat over $A$ in that case
May 5 19:48
but I have not worked this out
May 5 19:48
@BenSteffan I think you can get a counterexample with algebraic number theory by taking $B$ to the ring of integers of a number field with class number $>1$ and $M$ to be a fractional ideal which is not principal (but still a flat module) and then $A$ to be an order in the same number field such that $M$ divides the conductor...
May 5 19:34
and the Noetherian and f.g. assumptions seem superfluous in that case
May 5 19:34
@BenSteffan I think you need $\phi$ to be faithfully flat to get an iff
 
May 26 18:51
@theHigherGeometer the gap, if you wish to call it that is extremely straightforward to fill. I classified all possible ring structures in this situation and all are obviously commutative
May 26 18:51
@Enrico I don't understand your comment. The lemma is always about the same group: $\Bbb R$ with addition and its standard order and also in the question the addition and the order is fixed.
May 26 18:51
@Enrico I've spelled it out. Sometimes I like to leave some details to the reader. It's just an application of distributivity, as I alluded to.
May 26 18:51
@Enrico hope it's clearer now.
May 26 18:51
no worries, it's fine to ask if you don't understand something!
May 26 18:51
Why do you think that wlog doesn't fit? @Enrico
May 26 18:51
@Enrico I have not checked if this lemma appears somewhere in the literature, I thought it up myself for this answer, but I would hardly be surprised if it's written down somewhere.
May 26 18:51
@Enrico yes, I'm using the lemma twice.
 

­Trash

Where the trash goes.
May 6 11:56
@rschwieb would you mind deleting the message with my discordname?
May 5 21:35
@rschwieb you can add me on discord, my name is matheinboulomenos
 

 Math Mods' Office

For informal chat with the site moderators about moderation, s...
Jan 15 03:07
I think we should have some kind of policy on book recommendation questions. A significant amount of those is questions is closed as opinion based, but others are upvoted and don't receive a single close-vote. While some cases are clear, I don't see a pattern on which book recommendations are treated in which way. Actually, the top question in that tag is incredibly subjective and broad ("What books must every math undergraduate read?") with a score of 258 and no close-votes.
 

 CURED

For feedback/discussion/requests of Close/Undelete/Reopen/Edit...
Jan 8 01:25
I know there are already meta-posts about this, but in your opinion, what makes a [book-recommendation] question opinion-based or not opinion-based? It seems to me that some [book-recommendation] questions are closed as opinion-based while others receive lots of upvotes on the question and answers and not a single close vote and I don't really know what distinguishes these quetions