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<start_of_turn>user | |
summarize the following text: | |
Thank you very much to the organizers for inviting me to speak, which I guess came a | |
long time ago. | |
But I'm glad that we all made it here eventually. | |
Yeah, so I'm sorry about two things, the first is to break a long streak of very nice chalk | |
talk. | |
I'm afraid during COVID I got very lazy, so I just give talks on my iPad now. | |
The other thing I'm a bit sorry for is there won't really be chart degenerations in this | |
So yeah, I'll say kind of a couple of words, but really it's a hint to talk to other experts | |
in the audience about some chart degenerations arriving from these hospice centers. | |
So that said, what am I going to talk about? | |
Well, it's some joint work with Tasha Golashin, Tamek Lam, and David Speier. | |
And it's up on the archives, so the details are available. | |
Okay, so what is it? | |
Well, I'm going to talk about some varieties, type A braid varieties, and a particular combinatorial | |
structure on their coordinate rings, which is called a cluster structure. | |
So Nathan talked a little bit about what this means yesterday, I'll kind of refresh. | |
Okay, so it's a combinatorial structure, so there should be some combinatorial objects | |
in the background that give you this information, and those combinatorial objects we call 3D | |
playbit graphs. | |
It's little pictures that look like this. | |
Okay, and I guess I should say also, slides from my talk or some version of these slides | |
are on my website, so you don't have to worry about taking notes. | |
And okay, before I get going, I'll just say, so I'll say this a couple of times. | |
If you have a cluster structure on coordinate ring beer variety, you can, under certain | |
assumptions, produce chart degenerations. | |
And those chart degenerations, they really depend on this combinatorial gadget that you | |
cook up to give you the cluster structure. | |
So that's sort of the, if you maybe don't care about cluster alpha growths by themselves, | |
that's fine. | |
If you wanted to get your hands on any of these chart degenerations, you do need this | |
combinatorial information. | |
Okay, so first, let me just tell you about type A braid varieties. | |
So type A talk, so my G is SLN, and B is the subgroup of upper triangular matrices. | |
And as usual, I identified a quotient G mod B with the complete flag variety. | |
Okay, just to be really concrete, everyone has different conventions. | |
How do I go from a coset G B to a complete flag? | |
Well, the one dimensional subspace in my flag, that could be spanned by the first column | |
of G, so left most column. | |
The second, so the dimension two subspace, that's the left most two columns of G, etc. | |
Okay. | |
So to define these braid varieties. | |
I'm going to need the notion of relative position of two flags, which is a permutation. | |
So it's easier to describe when two flags are in relative position SI, so a simple transposition. | |
So F and F prime are in SI position if they differ in exactly the I subspace. | |
Great, so this transposition SI is telling you that subspace you have to change to get | |
from one to the other. | |
If you don't like thinking about flags, and instead you like thinking about cosets, well, | |
I think you're a little weird, that's okay. | |
There's also a way to phrase this implicit subspace. | |
Okay, so what about if two flags are in position W, where W is just a permutation, not necessarily | |
a simple transposition? | |
Well, there's this same coset condition, or the way I prefer to think about it, what this | |
means is, well, pick any reduced word for the permutation W that you want, then there's | |
a unique sequence of flags, which will interpolate. | |
So I interpolate between F and F prime, where, well, to go from F to the first flag in the | |
sequence, I change the I1 subspace, so as dictated by my choice of reduced word, and | |
then to go from F1 to F2, I change the I2 subspace, et cetera, et cetera, all the way | |
until I get to F prime. | |
So that's what being W related means. | |
Okay, so that said, let me say what type A grade varieties are. | |
Okay, so these are indexed by a pair. | |
The first element in my pair, I'm going to denote by beta underlined, and this is just | |
some word in simple transpositions, not necessarily reduced, so any word you want, and the other | |
element in my pair, U, is going to be a permutation that lives inside of this word somewhere. | |
So in this word, beta underlined, I have to be able to find a reduced expression for U. | |
Okay, so given that pair, here is my grade variety, X beta underlined U. | |
Okay, so a tuple of flags, which fit into a diagram like this. | |
So I like to think about these tuples of flags as walks. | |
So they're walks that start at the standard flag, and then in the first step of my walk, | |
I take, well, I take a step in direction SI1, so I change the I1 subspace, and then in the | |
second step, I change the I2 subspace, et cetera, all the way until I get to the last | |
one. | |
So this data of beta underlined is telling me the direction my walk has to move in. | |
And then this data of U is going to constrain where I end up. | |
So the exact way it constrains it is, my final flag in this walk, it has to be in relative | |
position W, not U, with the anti-standard flag. | |
Okay, this feels kind of weird, it's like, I will admit there are many more kind of natural | |
ways to use the data of U to constrain the final flag. | |
I'm making this choice to make connections with ridges and varieties. | |
I should say, okay, so another way to phrase this, which maybe uses words heard before, | |
well, if I just think about this space of walks where direction is dictated by beta | |
underlined, that's an open part of a Bot-Samuelson variety. | |
So in the Bot-Samuelson, in my walk, I could kind of like trip, or maybe I could be lazy. | |
So in my walk, I could not take a step at all. | |
So that would be an option in the full Bot-Samuelson, but I'm demanding that at each step of my | |
walk, I really change the flag. | |
So this is some open part. | |
This is the open part of Bot-Samuelson, and then there's a natural map from this open | |
part of Bot-Samuelson down to the flag variety, where I just look at the last flag in my walk. | |
And my grade variety is just the fiber over a particular opposite Schubert cell in the | |
flag variety of this map U projecting to the last flag. | |
So those are my, those are my grade varieties. | |
They were, well, not exactly these, but something quite close to these were studied by Laura | |
Escobar under the name of Brick Manifolds. | |
They're also kind of studied by Melick, who didn't call them anything. | |
And then Cathal, Gorstie, Gorstie, and Simonsall called them Brick Manifolds. | |
Any questions about the definition? | |
Oh, very good question. | |
The next point of the next slide. | |
Yeah. | |
So if you have two of these words. | |
So beta underline and gamma underline, and they're related by grade moves. | |
So either you're commuting like SI and SJ where I and J are far apart, or you're doing | |
the grade move. | |
So if they're related by grade moves, then the grade varieties are isomorphic. | |
So in fact, what I'm going to do is instead of indexing my grade varieties by the pair | |
of beta underline U, I'm just going to index it by beta U where beta is the element of | |
positive grade monoid represented by beta underline. | |
Positive grade monoid. | |
You don't know what that is. | |
It's not really important. | |
It's just, it's just like the symmetric group, except you throw away that SI squared equals | |
that. | |
So yeah. | |
So really what I'm trying to say is I have kind of a large isomorphism class of these | |
varieties and I really just kind of, well, yeah, I kind of want to think about them all. | |
So some, some things we know about them. | |
So some things we know about these varieties, well, they're smooth, they're irreducible, | |
they're affine or maybe quasi-affine if you want to be picky, and their dimension is | |
the length of beta minus the length of beta. | |
So there definitely are units. | |
There, there, there really are units on these, on these varieties. | |
So if you, if you feel a little bit like, okay, I mean, this is fine, but why are we | |
looking at these? | |
Let me give you a couple of examples, some nice examples, which I guess if you don't | |
know about the nice examples in advance, they won't be helpful, but if you do, hopefully | |
it's a motivation for examining. | |
So the, really the nicest case to think about is when your, or the kind of nicest special | |
case to think about is when your word beta underline is actually a reduced word. | |
So it's a reduced expression for some permutation B. Okay. | |
In that situation, having a walk like this between B and your final flag is actually | |
just saying that B and your final, final flag are in position B with each other. | |
That's all that it's saying. | |
So all of the information, like all of these flags, okay, and moreover, all of these flags | |
in the middle of the walk, they're actually completely determined by this flag FL plus | |
So in the situation where beta underline is reduced, you may as well just pay attention | |
to the final flag in your walk. | |
And so when you do that, you get an isomorphism from this kind of breed variety, just to a | |
subset of the full flag variety. | |
And what's the subset? | |
Well, it's just all the flag F that are in position B with the standard flag and position | |
W not U with the end of standard flag. | |
I mean, if you think about this a little bit harder, this is just an interesting example | |
of an intersection of a Schubert's cell and an opposite Schubert's cell inside the full | |
flag. | |
This is called an open Richardson's variety. | |
So I think they kind of come up in a lot of contexts, including total positivity and | |
work at loosing and breach. | |
Another interesting thing about them is their point counts over finite fields give you R | |
polynomials, which are important for computing Cauchon-Lustig polynomials. | |
So an even more special case of this nice special case. | |
Is if your permutation V has a unique descent, so beta underline is reduced expression for | |
this special kind of permutation V. In that situation, your grade variety is actually | |
isomorphic to a subset of a Grasmanian, so even better than being a subset of the full | |
It's isomorphic to what's called an open polythroid variety. | |
So these were, that name was coined and these were defined by Kutz and Lam and Spire, very | |
inspired by work of Kostnikov on polythroidism. | |
And I should also mention that Reach previously did work on Richardson's projected to Grasmanians | |
and other carbonyl flag varieties. | |
So I can't actually give you any real information about these open polythroid varieties. | |
What I will say is that there's a lot of really lovely combinatorics here. | |
And these open polythroid varieties, I think, are fairly well understood and worth of study | |
in their own right. | |
I'm just going to say, oh, they're a special piece of this thing that I'm going to talk | |
about. | |
And then kind of the very nicest open polythroid variety, well, or at least the biggest one, | |
is a particular subset of GRKN that I'm going to call GRKN-Circ. | |
And that's the subset of the Grasmanian where the cyclically consecutive Kluger coordinates | |
are known zero. | |
This came up in Nathan's talk yesterday. | |
So this is a special, very special example of one of these grades. | |
So those are the varieties I'll be dealing with. | |
And now let me say... | |
So that one there is the complementary divisor. | |
Mm-hmm. | |
But that's not true in general, I bet. | |
Yeah, it is. | |
So let me... | |
So... | |
Yes. | |
That's right. | |
But we're always going to be thinking about complementary divisors. | |
So that's what all the varieties will be. | |
Or inside the brick manifold, it's the complement of an anticononical divisor. | |
And what about here? | |
That is here. | |
So the... | |
And it also seems to be sort of... | |
Is it a sort of monomial divisor, I think, there, in the... | |
What's the question? | |
Super embedding, and... | |
So it's the coordinates. | |
We looked at it. | |
It's got P on them. | |
I'm not... | |
He says that in general. | |
I wonder whether it's in general as well. | |
So, I mean, you have to say what coordinates you're choosing here, but if you choose the | |
right coordinates, then I guess I'll tell you, yes. | |
Exactly. | |
Your coordinates are not always going to be... | |
They're not always going to be... | |
But I will... | |
I mean, the coordinates will be some frozen variable that across the structure... | |
Sorry. | |
I didn't get what you said. | |
It's a meta-theory keyword. | |
We're all going the same way. | |
Indeed. | |
So those are the varieties. | |
Now let me tell you what I'm going to tell you about them. | |
So what I'm going to do is... | |
So I pick a... | |
I fix a braid variety of interest. | |
So I activate a U. | |
And moreover, I pick, actually, a braid word for this braid. | |
So I pick a beta underline. | |
And from this... | |
<end_of_turn> | |
<start_of_turn>model |
Author
bigsnarfdude
commented
Feb 22, 2024
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