# Angry Birds Update

Angry birds peace treaty by Eretz Nehederet

A few years ago I became interested in the question of whather new versions of the computer game “Angry Birds” gradually makes it easier to get high scores. Devoted to the idea of Internet research activity I decided to explore this question on “ARQADE” a Q/A site for video games. I was especially encouraged by the success of an earlier question that was posted there by Andreas Bonini: Is Angry Birds deterministic? As you can see Bonini’s question got 239 upvotes making it the second most popular quastion in the site’s history. (The answer with 322 upvotes may well be the most popular answer!)  Is Angry Birds deterministic? (Click on pictures to enlarge.)

The question if Angry Birds is deterministic is the second most decorated question on Arqade, and its answers were extremely popular as well. (Other decorated questions include: How can I tell if a corpse is safe to eat? How can I kill adorable animals? and  My head keeps falling off. What can I do?.) As you can see from the comments taken from the site referring to science was warmly accepted!

## My question

I decided to ask a similar question about new versions and hoped for a similar success. Continue reading

# Two Delightful Major Simplifications

Arguably mathematics is getting harder, although some people claim that also in the old times parts of it were hard and known only to a few experts before major simplifications had changed  matters. Let me report here about two recent remarkable simplifications of major theorems. I am thankful to Nati Linial who told me about the first and to Itai Benjamini and Gady Kozma who told me about the second. Enjoy!

## Random regular graphs are nearly Ramanujan: Charles Bordenave gives a new proof of Friedman’s second eigenvalue Theorem and its extension to random lifts

Here is the paper. Abstract: It was conjectured by Alon and proved by Friedman that a random $d$-regular graph has nearly the largest possible spectral gap, more precisely, the largest absolute value of the non-trivial eigenvalues of its adjacency matrix is at most $2\sqrt{d-1} +o(1)$ with probability tending to one as the size of the graph tends to infinity. We give a new proof of this statement. We also study related questions on random n-lifts of graphs and improve a recent result by Friedman and Kohler.

## A simple proof for the theorem of Aizenman and Barsky and of Menshikov. Hugo Duminil-Copin and Vincent Tassion give  a new proof of the sharpness of the phase transition for Bernoulli percolation on $\mathbb Z^d$

Here is the paper Abstract: We provide a new proof of the sharpness of the phase transition for nearest-neighbour Bernoulli percolation. More precisely, we show that – for $p, the probability that the origin is connected by an open path to distance $n$ decays exponentially fast in $n$. – for $p>p_c$, the probability that the origin belongs to an infinite cluster satisfies the mean-field lower bound $\theta(p)\ge\tfrac{p-p_c}{p(1-p_c)}$. This note presents the argument of this paper by the same authors, which is valid for long-range Bernoulli percolation (and for the Ising model) on arbitrary transitive graphs in the simpler framework of nearest-neighbour Bernoulli percolation on $\mathbb Z^d$.

# Election Day

Today is the general election day in Israel, the third since starting this blog (I-2009,and II-2013). This is an exciting day. For me election is about participation much more than it is about influence and I try not to miss it. This time, for the first time,  I publicly supported one political party “the Zionist camp” headed by Herzog and Livni. The last four posts written in Hebrew are related to my position. (The fourth one has a poll which expresses also some academic curiosity, and I plan one more post with a similar follow-up  but post-election poll. But then we go back to combinatorics and more)

The blog also has now a new appearance and the header is a picture from 1999 with Jirka Matousek, a great mathematician and a great person who enlightened our community and our lives in many ways and who passed away at a terribly young age last Tuesday.

# From Oberwolfach: The Topological Tverberg Conjecture is False

The topological Tverberg conjecture (discussed in this post), a holy grail of topological combinatorics, was refuted! The three-page paper “Counterexamples to the topological Tverberg conjecture” by Florian Frick gives a brilliant proof that the conjecture is false.

The proof is based on two major ingredients. The first is a recent major theory by Issak Mabillard and Uli Wagner which extends fundamental theorems from classical obstruction theory for embeddability to an obstruction theory for r-fold intersection of disjoint faces in maps from simplicial complexes to Euclidean spaces. An extended abstract of this work is Eliminating Tverberg points, I. An analogue of the Whitney trick. The second is a result  by Murad Özaydin’s from his 1987 paper Equivariant maps for the symmetric group, which showed that for the non prime-power case the topological obstruction vanishes.

It was commonly believed that the topological Tverberg conjecture is correct. However, one of the motivations of Mabillard and Wagner for studying elimination of higher order intersection was that this may lead to counterexamples via Özaydin result. Isaak and Uli came close but there was a crucial assumption of large codimension in their theory, which seemed to avoid applying the new theory to this case.  It turned out that a simple combinatorial argument allows to overcome the codimension problem!

Florian’s  combinatorial argument which allows to use Özaydin’s result in Mabillard-Wagner’s theory  is a beautiful example of a powerful combinatorial method with other applications by Pavle Blagojević, Florian Frick and Günter Ziegler.

Both Uli and Florian talked about it here at Oberwolfach on Tuesday. I hope to share some more news items from Oberwolfach and from last week’s Midrasha in future posts.

# Midrasha Mathematicae #18: In And Around Combinatorics

Tahl Nowik

Update 3 (January 30): The midrasha ended today. Update 2 (January 28): additional videos are linked; Update 1 (January 23): Today we end the first week of the school. David Streurer and Peter Keevash completed their series of lectures and Alex Postnikov started his series.

___

Today is the third day of our winter school. In this page I will gradually give links to to various lectures and background materials. I am going to update the page through the two weeks of the Midrasha. Here is the web page of the midrasha, and here is the program. I will also present the posters for those who want me to: simply take a picture (or more than one) of the poster and send me. And also – links to additional materials, pictures, or anything else: just email me, or add a comment to this post.

# Scott Triumphs* at the Shtetl

Scott Aaronson wrote a new post on the Shtetl Optimized** reflecting on the previous thread  (that I referred to in my post on Amy’s triumph), and on reactions to this thread. The highlight is a list of nine of Scott’s core beliefs. This is a remarkable document and I urge everybody to read it. Yes, Scott’s core beliefs come across as feminist! Let me quote one of them.

7. I believe that no one should be ashamed of inborn sexual desires: not straight men, not straight women, not gays, not lesbians, not even pedophiles (though in the last case, there might really be no moral solution other than a lifetime of unfulfilled longing).  Indeed, I’ve always felt a special kinship with gays and lesbians, precisely because the sense of having to hide from the world, of being hissed at for a sexual makeup that you never chose, is one that I can relate to on a visceral level.  This is one reason why I’ve staunchly supported gay marriage since adolescence, when it was still radical.  It’s also why the tragedy of Alan Turing, of his court-ordered chemical castration and subsequent suicide, was one of the formative influences of my life.

!!

In the sacred tradition of arguing with Scott I raised some issues with #5 and 4# on Scott’s blog. Two of Scott’s points are on the subject of (young) people’s suffering by feeling unwanted, sexually invisible, or ashamed to express their desires.

I was pleased to see that those feminist matters that Scott and I disagree about, like the nature of prostitution, the role of feminist views in men’s (or nerdy men’s) suffering, and also Scott’s take on poverty, did not make it to Scott’s core beliefs.

Happy new year, everybody!

* The word triumph is used here (again) in a soft (non-macho) way characteristic to the successes of feminism. Voting rights for women did not exclude voting rights for men, and Scott’s triumph does not mean a defeat for  any others; on the contrary.

** “Shtetl-optimized” is the name of Scott Aaronson’s blog.

# @HUJI

Ilya Rips and me during Ilyafest last week (picture Itai Benjamini)

## Ilya Rips Birthday Conference

Last week we had here a celebration for Ilya Rips’ birthday. Ilya is an extraordinary mathematician with immense influence on algebra and topology. There were several startling ongoing mathematical projects that he is involved with that were discussed. One is a very ambitious project with Alexei Kanel-Belov is to get a “small cancellation theory” for rings and this has already fantastic consequences. Another is a work with Yoav Segev and Katrin Tent, on sharply 2- transitive groups, that answered a major old question with connections to groups, rings, and geometry. Happy birthday, Ilya!

## Achimedes on infinity

Reviel Netz (רויאל נץ) gave a seminar lecture in the department about infinity in Archimedes’ mathematical thoughts that developed into an interesting conversation. The lecture took place a day after Netz’s second poetry book (in Hebrew) appeared.

## The combinatorics school (midrasha) is coming.

Two weeks with extensive illuminating lecture series. Do not miss!

## At Combsem

On our Monday combinatorics seminar, we had, since my last report,  three excellent lectures. And next  Monday we are having Avi Wigderson.

Dec 1

Speaker: Sonia Balagopalan, HU

Title: A 16-vertex triangulation of the 4-dimensional real projective space