# Mathematics and Computation

## A blog about mathematics for computers

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# Posts in the category News

### Two PhD positions in Ljubljana starting October 2017

We are looking for two PhD students at the Faculty of Mathematics and Physics, University of Ljubljana. The programme starts in October 2017 and lasts three years. The positions will be fully funded (subject to approval by the funding agency). The candidates should have a Master’s degree in mathematics or computer science. No knowledge of Slovene is required.

The first PhD student will be advised by dr. Andrej Bauer. The topic of research is foundations of type theory. The candidate should have interest in mathematical aspects of type theory, and familiarity with proof assistants is desirable.

The second PhD student will be advised by dr. Matija Pretnar. The topic of research is the theory of programming languages with a focus on computational effects. The candidate should have interest in both the mathematical foundations and practical implementation of programming languages.

Candidates should send their applications as soon as possible, but no later than the end of April, to Andrej Bauer andrej.bauer@fmf.uni-lj.si or Matija Pretnar matija.pretnar@fmf.uni-lj.si, depending on their primary interest. Please include a short CV, academic record, and a statement of interest.

### Postdoc position in Ljubljana

A postdoc position in the Effmath research project is available at the University of Ljubljana, Faculty of Mathematics and Physics. The precise topic is flexible, but should generally be aligned with the project (see project description). Possible topics include:

•  implementation of computational effects
• proof assistants and formalization of mathematics

The candidate should have a PhD degree in mathematics or computer science, with background knowledge relevant to the project area. The position is available for a period of one year with possibility of extension, preferably starting in early 2016. No knowledge of the Slovene language is required.

The candidates should contact Andrej Bauer by email as soon as possible, but no later than January 8th 2016. Please include a short CV and a statement of interest.

### The HoTT book

The HoTT book is finished!

Since spring, and even before that, I have participated in a great collaborative effort on writing a book on Homotopy Type Theory. It is finally finished and ready for public consumption. You can get the book freely at http://homotopytypetheory.org/book/. Mike Shulman has written about the contents of the book, so I am not going to repeat that here. Instead, I would like to comment on the socio-technological aspects of making the book, and in particular about what we learned from open-source community about collaborative research.

### A hott thesis

Egbert Rijke successfully defended his master thesis in Utrecht a couple of weeks ago. He published it on the Homotopy type theory blog (here is a direct link to the PDF file (revised)). The thesis is well written and it contains several new results, but most importantly, it is a gentle yet non-trivial introduction to homotopy type theory. If you are interested in the topic but do not know where to start, Egbert’s thesis might be perfect for you. As far as I know it is the first substantial piece of text written in (informal) homotopy type theory.

What I find most amazing about the work is that Egbert does not have to pretend to be a homotopy type theorist, like us old folks. His first contact with type theory was homotopy type theory, which impressed on his mind a new kind of geometric intuition about $\Pi$’s, $\Sigma$’s and $\mathrm{Id}$’s. If we perform enough such experiments on young bright students, strange things will happen.

### Eff 3.0

Matija and I are pleased to announce a new major release of the eff programming language.

In the last year or so eff has matured considerably:

• It now looks and feels like OCaml, so you won’t have to learn yet another syntax.
• It has static typing with parametric polymorphism and type inference.
• Eff now clearly separates three basic concepts: effect types, effect instances, and handlers.
• How eff works is explained in our paper on Programming with Algebraic Effects and Handlers.
• We moved the source code to GitHub, so go ahead and fork it!

### Bob Harper has a blog

Bob Harper of CMU, has recently started a blog, called Existential Type, about programming languages. He is a leading expert in Programming Languages. I remember being deeply inspired the first time I heard him talk. I was an incoming graduate student at CMU and he presented what the programming languages people at CMU did. His posts are fun to read, unreserved and very educational. Highly recommended!

### European workshop on computational effects

Alex Simpson, Matija Pretnar and I are organizing a workshop on computational effects. It will take place in Ljubljana on March 17th and 18th 2011. More information is available at the workshop web page.

### An amazing functional

Martin Escardo and Paulo Oliva have been working on the selection monad and related functionals. The selection monad S(X) = (X -> R) -> X is a cousin of the continuation monad C(X) = (X -> R) -> R and it has a lot of useful and surprising applications. I recommend their recent paper “What Sequential Games, the Tychonoff Theorem and the Double-Negation Shift have in Common” which they wrote for MSFP 2010 (if you visit the workshop you get to hear Martin live). They explain things via examples written in Haskell, starting off with the innocently looking functional ox (which i I am writting as ox in Haskell for “crossed O”):

ox :: [(x -> r) -> x] -> ([x] -> r) -> [x]
ox [] p = []
ox (e : es) p = a : ox es (p . (a:))
where a = e (\x -> p (x : ox es (p . (x:))))

It is just four lines of code, so how complicated could it be? Well, read the paper to find out. If you are ready for serious math, have a look at this paper instead.

### Hardware failure and phishing attacks

After more than 1300 days of uninterrupted service, the good old PC that served the blog started to spontaneously reboot every 4 minutes or so. It looks like a hardware failure. I moved the site to a temporary machine. I am seriously considering renting a private virtual server and just forget about buying my own hardware in the future.

On top of that I discovered that evil forces planted a phishing attack on the blog about two weeks ago. The strategy was this:

1. Create an account on my blog (I stupidly left registration open to everyone).
2. Elevate account privileges to administrator by exploiting a WordPress security hole (I do not know which one).
4. Direct phishing victims to the uploaded files.

So, keep your WordPress as closed as possible.

### A comment about “Mathematical undecidability and quantum randomness” by Tomasz Paterek et al.

This is a short note pointing out that the recent paper on“Mathematical undecidability and quantum randomness” by Tomasz Paterek et al. is no black magic, and that the authors are well aware of it. Unfortunately the paper appeared on Slashdot and has since generated an infinite amount of quasi-mathematical discussions.

### Exact real arithmetic in Haskell

HERA is an implementation of exact real arithmetic in Haskell using the approach by Andrej Bauer and Iztok Kavkler, see these and these slides. It uses the fast multiple precision floating point library MPFR. Download source, and see documentation and examples of usage at my home page.

[Note by Andrej: this is a guest post by Aleš Bizjak, a first-year student of mathematics at my department. I am very proud of the excellent work he did on his summer project.]

### Lambda calculus for real analysis by Paul Taylor

Paul Taylor has published a revised version of his lambda-calculus for real analysis. I recommend it to anyone who is interested in real analysis, be it a computer scientist, numerical analyst, or just a “true” analyst.

The first, second, and third time I talked to Paul I could not understand a word of what he was saying, and that’s not just because he is a native speaker of English English. I only began to “get it” when he visited me in Ljubljana. So I think it’s perhaps worth explaining a bit what this “lambda-calculus for real analysis” is about.