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#combinatorics

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A fundamental result in universal algebra is the Subdirect Representation Theorem, which tells us how to decompose an algebra \(A\) into its "basic parts". Formally, we say that \(A\) is a subdirect product of \(A_1\), \(A_2\), ..., \(A_n\) when \(A\) is a subalgebra of the product
\[
A_1\times A_2\times\cdots\times A_n
\]
and for each index \(1\le i\le n\) we have for the projection \(\pi_i\) that \(\pi_i(A)=A_i\). In other words, a subdirect product "uses each component completely", but may be smaller than the full product.

A trivial circumstance is that \(\pi_i:A\to A_i\) is an isomorphism for some \(i\). The remaining components would then be superfluous. If an algebra \(A\) has the property than any way of representing it as a subdirect product is trivial in this sense, we say that \(A\) is "subdirectly irreducible".

Subdirectly irreducible algebras generalize simple algebras. Subdirectly irreducible groups include all simple groups, as well as the cyclic \(p\)-groups \(\mathbb{Z}_{p^n}\) and the Prüfer groups \(\mathbb{Z}_{p^\infty}\).

In the case of lattices, there is no known classification of the finite subdirectly irreducible (or simple) lattices. This page (math.chapman.edu/~jipsen/poset) by Peter Jipsen has diagrams showing the 92 different nontrivial subdirectly irreducible lattices of order at most 8. See any patterns?

We know that every finite subdirectly irreducible lattice can be extended to a simple lattice by adding at most two new elements (Lemma 2.3 from Grätzer's "The Congruences of a Finite Lattice", arxiv.org/pdf/2104.06539), so there must be oodles of finite simple lattices out there.

A question for the (combinatorial) hive mind.

There are a lot of extremal results that are matched asymptotically by some probabilistic construction, but with some gap, often quite substantial. I'm thinking about the Ramsey numbers R(k,k) or R(3,k), but examples of this phenomenon are prevalent.

I'm curious, does someone out there know of good examples of (extremal) results where some probabilistic construction (e.g. via a random graph) is matched asymptotically, and very precisely?

A post of @11011110 has reminded me that (after a year and a half lurking here) it's never too late for me to toot and pin an intro here.

I am a Canadian mathematician in the Netherlands, and I have been based at the University of Amsterdam since 2022. I also have some rich and longstanding ties to the UK, France, and Japan.

My interests are somewhere in the nexus of Combinatorics, Probability, and Algorithms. Specifically, I like graph colouring, random graphs, and probabilistic/extremal combinatorics. I have an appreciation for randomised algorithms, graph structure theory, and discrete geometry.

Around 2020, I began taking a more active role in the community, especially in efforts towards improved fairness and openness in science. I am proud to be part of a team that founded the journal, Innovations in Graph Theory (igt.centre-mersenne.org/), that launched in 2023. (That is probably the main reason I joined mathstodon!) I have also been a coordinator since 2020 of the informal research network, A Sparse (Graphs) Coalition (sparse-graphs.mimuw.edu.pl/), devoted to online collaborative workshops. In 2024, I helped spearhead the MathOA Diamond Open Access Stimulus Fund (mathoa.org/diamond-open-access).

Until now, my posts have mostly been about scientific publishing and combinatorics.

#introduction
#openscience
#diamondopenaccess
#scientificpublishing
#openaccess
#RemoteConferences
#combinatorics
#graphtheory
#ExtremalCombinatorics
#probability

igt.centre-mersenne.orgInnovations in Graph Theory Innovations in Graph Theory

New account, new introduction!

I'm Beth. I'm a queer mathematician who loves musical theater, webcomics, teaching math, and my cat.

Favorite areas of math: Topology, geometry, and combinatorics.

Favorite musicals: Chess, Into the Woods, Next to Normal, Sunday in the Park With George, Sweeney Todd

Favorite webcomics: this is long enough to get its own post:
https://transfem.social/notes/9ius8efgn12cewcz

#Introduction #Queer #Mathematician #Math #Musicals #MusicalTheater #MusicalTheatre #Webcomics #Teaching #TeachingMath #Cat #Cats #SillyGoose #Topology #Geometry #Combinatorics #Chess #ChessTheMusical #IntoTheWoods #NextToNormal #SundayInTheParkWithGeorge #SweeneyTodd #PandorasTaleWiki #RainverseWiki

TransFem Space@183231bcbThis is a selection of my favorite #WebComics. They are in alphabetical order, NOT order of preference. Alice and the Nightmare https://www.aliceandthenightmare.com/ #AliceAndTheNightmare Castoff https://castoff-comic.com/ https://tapas.io/series/Castoff https://www.webtoons.com/en/challenge/castoff/list?title_no=50984 #Castoff Cosmic Fish https://cosmicfishes.com/ #CosmicFish Dumbing of Age https://www.dumbingofage.com/ #DumbingOfAge Heartless https://heartless-comic.com/ #heartless Heroes of Thantopolis https://www.heroesofthantopolis.com/ #HeroesOfThantopolis Inhibit by @evegwood@mastodon.social https://inhibitcomic.com/ https://tapas.io/series/Inhibit/info https://www.webtoons.com/en/challenge/inhibit/list?title_no=40462 #Inhibit Kiwi Blitz https://www.kiwiblitz.com/ #KiwiBlitz Lefty Cartoons https://leftycartoons.com/ #LeftyCartoons Lies Within https://www.lieswithincomic.com/ #LiesWithin Monster Lands https://monster-lands.com/ #MonsterLands Namesake https://www.namesakecomic.com/ #Namesake Never Heroes https://www.neverheroescomic.com/ #NeverHeroes Never Satisfied https://www.neversatisfiedcomic.com/ #NeverSatisfied Nix of Nothing by @MLeeLunsford@mastodon.social https://www.nixofnothing.com/ #NixOfNothing No End https://noendcomic.com https://tapas.io/series/No-End/info https://www.webtoons.com/en/canvas/no-end/list?title_no=909579 #NoEnd Novae https://www.novaecomic.com/ https://www.webtoons.com/en/challenge/novae/list?title_no=106507 #Novae Pandora's Tale by @comicsbyxan https://pandorastale.com/ https://tapas.io/series/Pandoras-Tale https://www.webtoons.com/en/challenge/pandoras-tale/list?title_no=287377 #PandorasTale Pathways: Chronicles of Tuvana http://pathways.elainetipping.com/ https://tapas.io/series/Pathways-Chronicles-of-Tuvana https://www.webtoons.com/en/challenge/pathways-chronicles-of-tuvana/list?title_no=128848 #PathwaysChroniclesOfTuvana Radio Silence http://www.radiosilencecomic.com/ https://www.webtoons.com/en/challenge/radio-silence/list?title_no=35242 #RadioSilence Rain by @LynnSenpai@retro.pizza https://rain.thecomicseries.com/ #rain Recoil https://recoil.one/ https://www.webtoons.com/en/challenge/recoil/list?title_no=256833 #recoil Saints' Quarter https://saintsquarter.com/ https://tapas.io/series/SaintsQuarter https://www.webtoons.com/en/challenge/saintsquarter/list?title_no=65944 #SaintsQuarter Sister Claire https://www.sisterclaire.com/ #SisterClaire Skyvein https://www.skyveincomic.com/ https://tapas.io/series/skyvein https://www.webtoons.com/en/challenge/skyvein/list?title_no=651351&webtoon-platform-redirect=true #skyvein Sleepless Domain https://www.sleeplessdomain.com/ #SleeplessDomain Slightly Damned https://www.sdamned.com/ #SlightlyDamned Speak of the Devil by @MLeeLunsford@mastodon.social https://www.speakdevil.com/ #SpeakOfTheDevil StarHammer https://www.starhammercomic.com/ #StarHammer Suihira: The City of Water http://suihira.com/ https://tapas.io/series/Suihira https://www.webtoons.com/en/challenge/suihira-the-city-of-water/list?title_no=39385 #suihira Supernormal Step by @MLeeLunsford@mastodon.social https://www.supernormalstep.com/ #SuperNormalStep Tamberlane https://www.tamberlanecomic.com/ https://www.webtoons.com/en/challenge/tamberlane/list?title_no=372674 #tamberlane The End http://www.endcomic.com/ #TheEnd The Order of the Stick https://www.giantitp.com/ #TheOrderOfTheStick The Property of Hate https://jolleycomics.com/TPoH/The_Hook/1 #ThePropertyOfHate The Psions https://tapas.io/series/The-Psions #ThePsions The Roommate From Hell http://enchantedpencil.com/roomie/ https://tapas.io/series/The-Roommate-From-Hell https://www.webtoons.com/en/challenge/the-roommate-from-hell/list?title_no=206046 #TheRoommateFromHell The Sanity Circus https://www.sanitycircus.com/ #TheSanityCircus #SanityCircus This Is Not Fiction https://www.thisisnotfiction.com #ThisIsNotFiction Tigress Queen https://www.tigressqueen.com/ #TigressQueen Widdershins https://widdershinscomic.com/ #widdershins

During our #combinatorics in #physics online conference, we have interactively made a mindmap to connect speakers, audience, and shared topics. Besides looking funny, this has really helped me concentrate on the talks because I had to pay attention if something had been mentioned in a previous talk already.
A full resolution version is on my website.

🔴 A numerical evaluation of the Finite Monkeys Theorem

"From this, we can see that all but the most trivial of phrases will, in fact, almost certainly never be produced during the lifespan of our universe. There are many orders of magnitude difference between the expected numbers of keys to be randomly pressed before Shakespeare's works are reproduced and the number of keystrokes until the universe collapses into thermodynamic equilibrium..."

Woodcock, S. and Falletta, J. (2024) 'A numerical evaluation of the Finite Monkeys Theorem,' Franklin Open, p. 100171. doi.org/10.1016/j.fraope.2024..

doi.orgA numerical evaluation of the Finite Monkeys TheoremThe Infinite Monkeys Theorem has long-established the eventual certainty of the complete works of William Shakespeare being reproduced by a monkey ran…

Our #workshop on #combinatorics in fundamental #physics has three topic days, November 26-28.

Day 1: Random Geometry for #Quantum #Gravity

Random geometry is a powerful mathematical framework for studying quantum gravity by modeling it as a statistical physics system, where each Boltzmann configuration corresponds to a spacetime geometry selected from an ensemble with a well-defined probability measure. When considering quantum gravity from a lattice perspective, where spacetime is discrete, the challenge of defining a suitable probability measure becomes a combinatorial problem.

Day 2: #Causal Set Theory

Causal Set Theory is an approach to Quantum Gravity in which spacetime is fundamentally discrete and takes the form of a locally finite partial order, or causal set. The twin questions leading much of the research in this field are: How does the continuum physics of General Relativity arise from an underlying discreteness? And what is the quantum nature of a discrete and dynamical spacetime? #causalset

Day 3: Combinatorics in Perturbative #QFT

A unique feature of quantum field theory is the central role that combinatorics plays: from generating Feynman graphs of scalar models to combinatorial maps and higher graph-like objects of sophisticated frameworks such as matrix-/tensor- and group field theories to renormalization Hopf algebras, and the theory of resurgence and asymptotic power series, to name but a few. The focus of this workshop will be on matrix-/tensor-/group-field theories and graph complexes.

Registration is open for everyone, and of course, free of charge.
indico.mitp.uni-mainz.de/event

MITP (Indico)YOUNGST@RS - Combinatorics in Fundamental PhysicsCombinatorics, a branch of mathematics with powerful applications in modern physics, plays a significant role in fundamental areas such as enumerating Feynman diagrams and addressing discreteness in various approaches to Quantum Gravity. This workshop will highlight connections across different fields, encourage collaboration, and contribute to setting new scientific targets for the community. The workshop is designed to build new bridges across adjacent disciplines, encouraging the exchange...

The registration to our #combinatorics in #physics workshop is finally open! November 26-29, virtual on zoom hosted by #MITP Mainz.
indico.mitp.uni-mainz.de/event
From the intro: "The workshop is designed to build new bridges across adjacent disciplines, encouraging the exchange of ideas and the transfer of advanced mathematical tools from pure combinatorics to physics. By bringing together experts and young researchers, it aims to inspire mathematicians to explore topics relevant to physicists and facilitate interdisciplinary collaboration."

MITP (Indico)YOUNGST@RS - Combinatorics in Fundamental PhysicsCombinatorics, a branch of mathematics with powerful applications in modern physics, plays a significant role in fundamental areas such as enumerating Feynman diagrams and addressing discreteness in various approaches to Quantum Gravity. This workshop will highlight connections across different fields, encourage collaboration, and contribute to setting new scientific targets for the community. The workshop is designed to build new bridges across adjacent disciplines, encouraging the exchange...

#Mathober #Mathober2024

The prompt for day 5 was 'Integer Partitions'. In number theory, a partition is a way to write an integer as a sum of positive integers. For example, there are 5 partitions of 4, given by 1+1+1+1, 2+1+1, 2+2, 3+1 and 4. The partition function is the name given to the function that counts how many partitions a given integer has. So the above examples show that p(4)=5. The first few values of the partition function are 1, 1, 2, 3, 5, 7, 11, ....

In 1937, Hans Rademacher found a complicated formula for the partition function in the form of an infinite series. You can see the full formula here en.wikipedia.org/wiki/Partitio. One interesting feature of this formula is that it allows you to calculate a value of p(x) even when x is not an integer. This was explored and graphed a bit by Fredrik Johansson over at mathoverflow.net/questions/366. He points out that when x is n+1/2 for natural n, the infinite series is zero for every term except the first. This then gives you a closed form expression, which he doesn't actually write out because it's awful:

p(x) = (√(2/3)cosh(π√(2/3)√(x-1/24)) - sinh(π√(2/3)√(x-1/24))/(π√(x-1/24)))/(2√2(x-1/24))

It's interesting that p(n+1/2) has this closed form formula, because no such formula is known for p(n) itself.

Of course it would be quite irresponsible to say 'There are 0.8458... ways to write 1/2 as the sum of natural numbers', so I won't.

I posted a new paper on the arXiv!

arxiv.org/abs/2409.12923

In "Higher-dimensional book-spaces" I show that for each \(n\) there exists an \(n\)-dimensional compact simplicial complex which is a topological modular lattice but cannot be endowed with the structure of topological distributive lattice. This extends a result of Walter Taylor, who did the \(2\)-dimensional case.

I think this kind of result is interesting because we can see that whether spaces continuously model certain equations is a true topological invariant. All of the spaces that I discuss here are contractible, but only some can have a distributive lattice structure.

A similar phenomenon happens with H-spaces. The \(7\)-sphere is an H-space, and it is even a topological Moufang loop, but it cannot be made into a topological group, even though our homotopical tools tell us that it "looks like a topological group".

This is (a cleaned up version of) something I did during my second year of graduate school. It only took me about six years to post it.

arXiv.orgHigher-dimensional book-spacesIn 2017, Walter Taylor showed that there exist $2$-dimensional simplicial complexes which admit the structure of topological modular lattice but not topological distributive lattice. We give a positive answer to his question as to whether $n$-dimensional simplicial complexes with the same property exist. We do this by giving, for each $n\ge2$, an infinite family of compact simplicial complexes which admit the structure of topological modular lattice but not topological distributive lattice.

🤝 As every year, the @LipnLab will support and help candidates for the #CNRS "Chargé·e de recherche" application. These are tenured full-time research positions for Junior Researchers that can evolve into Research Director positions.
⏱️ The deadline is late January, (TBA) but you do need to contact one of our team well in advance. Have a look!
🔗 lipn.univ-paris13.fr
📝 Guiding document for applications:
🔗 cn6.fr/documents/criteria.pdf
#ML #Combinatorics #optimization #NLP #HPC #Logic #Verification

lipn.univ-paris13.frLIPN – Laboratoire d'Informatique de Paris Nord

I wanted to share it one day. I haven't read many good #math books. But those few good ones are REAL good. One of them being "Mathematics of choice". When I was presented with combinatorics topic in school, I could not understand a thing. But with this #book, it made me understand #combinatorics for real and love this branch of math! By the way, if you are reading some topic, and you don't understand, it might be that you've chosen the wrong book or article to read. Not because you are dumb.

I have a new YouTube channel! You can find the intro video for it at youtu.be/3jmO0IixHZw

Make sure you subscribe to this one if you want to see my future Math Research Livestream videos, or any other video I post in the future.

You can find a video explaining the change on my old channel (youtu.be/PCBvt7vNB8I). The tl;dr is that my old account was tied to my undergrad email, and I want something which is not managed by another organization.

I'll be leaving all the old videos up so that links don't die, and I will also repost them to my new account over the coming weeks.