Relationship between Cahen’s constant and who you’re gonna call

Since a while back I’ve used weekly, small, well-defined programming exercises for all my courses. It’s a lot of work to develop these, but also a lot of fun.

The book I’m using, Sedgewick and Wayne’s Algorithms (4th ed.), makes it natural to have an exercise about the four sum problem early on, involving a bit of coding, parsing, simple experimental and theoretical running time analysis, and a clever idea or two. Rasmus Pagh had a brilliant idea how to turn this exercise into something fun, inspired by xkcd 687. After all, finding deep-looking product formulas is just a question of looking (after taking logarithms) for values that add to 0. Given enough interesting numbers, some pattern has to emerge.

I’ve played around with Rasmus’s exercise it a bit more since yesterday, added some spit and polish, and made it slightly crazier. The best new law of physics I’ve discovered this way is shown at the top. It relates

• the energy equivalent m_uc² of the atomic mass constant in in MeV,
• the energy equivalent m_dc² of the mass of a deuterium atom in Joule,
• Cahen’s constant (roughly 0.643410), defined by a sum of Sylvester numbers a_k, and
• G, the phone number of the Ghostbusters, 5552368.

You can’t make that up. It’s correct up to rounding errors and a cavalier attitude to dimensions.

The finished exercise, with data files, is at the course blog at ITU; I’m slowly getting myself organised to publishing these things on GitHub to make them more useful for other teachers. (That would include the TeX file for the exercise description, for example.) Stay tuned.

Edit to add: I guess the real conclusion of this formula is that the world finally knows the unit and dimension of US phone numbers, assuming that Cahen’s constant is dimension-less. Phone numbers express inverse energy squared, measured in mega electron Volt Joule. You heard it here first.

Illustration of Yates’s algorithm from my talk. I finally found a way to simultaneously depict the lattice structure and the symmetries of the circuit.

Many computer science fields in France are organised in national, cross-institutional groupes de recherches, not completely unlike the ACM SIGs in the US. The TCS group (“Mathematical Informatics”) encompasses some 400-500 people, as far as I understand.

As in most of Europe, theoretical computer science in France includes many more fields more than US-style theory of computation. Amazingly, they meet once a year for two days, and give well-attended talks to each other. The 2012 meeting had 170 registrants, an impressive number.

This struck me as particularly noteworthy after just attending SODA, where the various subfields of algorithms become increasingly fragmented and estranged, to the point of hostility and mutual incomprehensibility.

At the Paris meeting, a steering committee selects a number speakers from the various working groups in the GDR IM, who give meaty, 45-minutes talks to a general TCS audience: Computational logic, computational geometry, distributed systems, process calculi, extremal graphs, ….

In addition, the meeting includes two 1-hour invited talks recruited outside of the French GdR. Ashwin Nayak talked about communication complexity, and I used to opportunity to present an overview of zeta transform algorithms and applications, culminating in our SODA 2012 result from last week. [slides]

Thanks to everybody who attended, and to the nice organisers for putting me into a disarmingly charming hotel in the middle of the Latin quarter, where you couldn’t swing a dead Marsipulami without hitting a comic store. I had a splendid time.

“Coded cultural choice” on the cover of SvD’s Arts section, illustrated by badly indented Java code.

The article is about the effect of algorithms on the consumption of culture. One of the arguments that I’ve been trying to make for a while is that (a) the human condition is increasingly determined by access to information (b) the main force affecting information is algorithms. Thus algorithms are a worth understanding for other reasons than (a) intellectual curiosity or (b) technological progress.

The web version of the article is online at svd.se.

Such an article are the result of an interview that takes over an hour, and where I say a lot of things about a lot of things, and some subsequent emails. What ends up in the final article is a bit of hit and miss. This time, I’m pretty happy with the outcome.

Some highlights and comments:

• My physics envy is in full display: “We teach 1960’s science like physics and chemistry in school, but you don’t learn more about algorithms now than 20 years ago.” This statement is of course not universally true; many countries do have programming and other computer science as part of the high school curriculum. But even with that proviso I think it’s safe to say that “what science should be thought in school” has changed relatively little since Sputnik. But the point of the article is of course something else: algorithms have an effect on culture (and democracy and whatnot), so the bold claim is that algorithmic thinking should be part of the Social Science curriculum. (It goes without saying that recursion ought to stand proudly next to Pascal’s law in the Science and Maths curriculum as well.)
• The article contains a nice-looking attempt at actually Explaining What’s Going On. You can see some pseudocode in the web version, where nice-looking brand icons serve as objects that are polled for various signals about previous user behaviour. There is even a collaborative filtering aspect (since the hypothetical Facebook object is queried about what your friends like). Given the constraints under which such a page is produced, I think this came out well. The real point is of course that this code is not written by a human, but by another algorithm, so “the algorithm” performs at a meta-level. I did not find a good way to communicate that, so I didn’t try.
• The code on the section’s front page is from the very nice data mining research done at ITU: Andrea Campagna and Rasmus Pagh, Finding Associations and Computing Similarity via Biased Pair Sampling, The Ninth IEEE International Conference on Data Mining, Miami, Florida, USA, 6-9 December 2009 pp 61-70. Proper indentation or choice of a non-proportional font is not something the newspaper graphics guy cares about. (Hell, my own students don’t.) I proudly showed this to ITU’s vice chancellor Mads Tofte, but he “would have been more impressed if it had been in ML.”
• Lest somebody misattributes pithy quotations to quotable me, the phrase “program or be programmed” is from Douglas Rushkoff.

The European Association for Theoretical Computer Science has around 1000 members, including many European TCS researchers and everybody who attended conferences like ICALP and ESA in a particular year. Traditionally, EATCS has had a close relationship to scientific publisher Springer, who publishes the proceedings of EATCS’s “flagship conference” ICALP.

A recurring theme at the EATCS general assembly meetings for the past many years has been the publication model for ICALP. Should “we” continue with Springer, or move to an open-access publisher like LIPIcs? After considerable deliberation, a ballot among EATCS members was held in late 2011.

For more background:

1. EATCS general assembly at ICALP 2010 on my blog.
2. EATCS general assembly at ICALP 2011 on my blog.
3. EATCS ballot on Luca Aceto’s blog.

I have been absolutely thrilled that the EATCS council decided to send this question to its members. Well done. I want to see more of this.

Unfortunately, the outcome is not what I would have hoped. The EATCS council required a quorum of 25% of EATCS members for the ballot to be valid. I think this is a reasonable requirement – the ICALP publishing model is an important decision that should be safely moored.

Alas, the quorum was not reached. On 9 November 2011, EATCS president Burkhard Monien informed the EATCS that out of the 1094 members, only 260 had voted. And that’s just slightly less than 1 out of 4. It’s 23.8%.

Informative graphic explaining the magnitude of the value 13 in relation to the value 1094

Monien adds,

However, I also want to let you know that the majority of the 260 votes were in favor of LIPIcs. EATCS will observe the further development carefully.

So at least we know the opinion of those EATCS member who care. Next time we just need to remember to inform intelligent adults of the importance to actually vote.

A 9-input sorting network in action

IT University of Copenhagen opened its doors to the public last Friday in connection with the city-wide “Culture Night”, so I used the opportunity to implement the Sorting Networks activity.

ITU has a big stair case in its atrium, which had the proper size for a 9-input sorting network. I measured everything up, found a good-looking network in Knuth’s The Art of Computer Programming, and sketched the layout for the friendly folks at Facilities Management.

The network was “drawn” on the stairs using adhesive tape, which took a few hours on the day before the event. It looked absolutely splendid. The activity was manned with enthusiastic undergraduate students for several hours, who herded eager and intrigued guests of all ages through the maze of comparators. A smashing success.

Also, a student volunteered to translate the Wikipedia page on sorting networks into Danish, so the outreach activity had a permanent, digital result as well: Sorteringsnetværk.

Originally, I had also promised myself to produce a large poster or two next to the activity, which should explain what’s actually going on, why sorting is important, and (in particular) show a few more concrete networks of various sizes. This would have given the whole activity a bit more scientific muscle. Alas, I failed to get this done in time. Better next time.

I also failed to show up in time for the kick-off, instructing the student volunteers in how the whole thing was supposed to work. Apparently, they spent a good number of attempts running the network in the reverse direction (from the top of the stairs and down, which arguably is the obvious direction), and tried to find the bug! So now they know that sorting networks don’t necessarily work in both directions. (But insertion sort does!) This may be a fresh research question: find an optimal bidirectional sorting network for 9 inputs.

All in all, a splendid event. Thanks to everybody who helped, or just wanted to get themselves sorted out.

Based on my quick perusal of list of accepted papers [PDF], here’s a list of papers related to exponential time computation, together with references to online version — I’m probably missing some. Updates are welcome.

• Counting Perfect Matchings as Fast as Ryser [arxiv.org 1107.4466], by Andreas Björklund
• Fixed-Parameter Tractability of Directed Multiway Cut Parameterized by the Size of the Cutset, by Rajesh Chitnis, Mohammadtaghi Hajiaghayi and Dániel Marx
• Co-nondeterminism in compositions: A kernelization lower bound for a Ramsey-type problem [arxiv 1107.3704], by Stefan Kratsch
• Weak Compositions and Their Applications to Polynomial Lower-Bounds for Kernelization [ECCC 2011-072], by Danny Hermelin and Xi Wu
• Subexponential Parameterized Algorithm for Minimum Fill-in [arxiv 1104.2230] by Fedor V. Fomin and Yngve Villanger
• A Satisfiability Algorithm for AC₀ [arxiv 1107.3127], by Russel Impagliazzo, William Matthews and Ramamohan Paturi
• Compression via Matroids: A Randomized Polynomial Kernel for Odd Cycle Transversal [arxiv 1107.3068] by Stefan Kratsch and Magnus Wahlström
• Fast zeta transforms for point lattices, by Andreas Björklund, Thore Husfeldt, Petteri Kaski, Mikko Koivisto, Jesper Nederlof and Pekka Parviainen
• Shortest Cycle Through Specified Elements [PDF], by Andreas Björklund, Thore Husfeldt and Nina Taslaman
• Kernelization of Packing Problems, by Holger Dell and Dániel Marx
• Linear Kernels for (Connected) Dominating Set on H-minor-free graphs [PDF], by Fedor V. Fomin, Daniel Lokshtanov, Saket Saurabh and Dimitrios Thilikos
• Bidimensionality and Geometric Graphs [arxiv 1107.2221], by Fedor V. Fomin, Daniel Lokshtanov and Saket Saurabh

An amazing number of kernelisation (and, I presume non-kernelisation) papers. And lots and lots of exciting papers in many other fields as well.

Computer Sweden, 19 Aug 2011, page 2.

A Swedish colleague made me aware that I am featured in some kind of quote of the week in Swedish IT newspaper Computer Sweden.

On du utnyttjar en tjänst som du inte behöver betala för är det inte du som är kunden, då är det du som är varan.

Now, in the words of Alexander Smith, to be occasionally quoted is the only fame I care for, so I’m really happy. This is a step in the right direction towards a meaty, forthcoming collection of Thore’s Best Aphorisms or The Quotable Husfeldt or something like that.

(In fact, the quote doesn’t sound like very good Swedish to me. Seeing something you said quoted is strange already. Seeing something you said in your fourth language is even stranger.)

The quote must come from a recent interview with Svenska Dagbladet inspired by the filter bubble, and is aimed at Google’s business model. It goes without saying that there are plenty of free IT products, in particular from the open source community, that don’t view me as a product. Syntactic awkwardness and quibbles of generality aside, I think it’s a good quote.

In the words of Monty Python’s Oscar Wilde sketch, I wish I had said it.

Here’s the source:

If you’re not paying for something, you’re not the customer; you’re the product being sold.

The quote opens chapter 1 of Eli Pariser’s book The Filter Bubble and is attributed to Andrew Lewis, under the handle blue_beetle. Much pithier. In fact, the quoted website metafilter.com gives the quote as “[…] paying for it […]”.

(Of course, Wilde probably already said it a century ago, and even better.)

Attendee at ICALP 2011 business meeting. The image shows the free ice cream and name tag.

This was a very well-organised event hosted at ETH. Michael Hoffmann had everything under complete control and remained confident, alert, calm and accessible. I’ve tried go take a glimpse behind the curtains, such as looking at the two-page to-do list for technical staff in the lecture halls, and the level of detail and relevance is just off the scale. I hope Michael can summarise his preparation and tips for everybody’s benefit.

Future Publication of the ICALP Proceedings

The EATCS general assembly is by tradition a long and dry event without free beer. To break new ground, the organisers handed out free ice cream. ’tis not beer, but a welcome step in the right direction. I hope the organisers of next year’s ICALP, Warwick, take note.

Burkhard Monien’s EATCS Annual report 2010-2011 [PDF] is available on-line, so there’s no need to summarise it here.

The exciting question is the publication model for ICALP: will the EATCS flagship conference stay with Springer’s LCNS series (or some transformation of it) or not? I blogged about this last year, from the 2010 assembly:

Monien explained the EATCS council wants to get this right, ICALP is a big steamer, should not go in zig-zag course, and stick to its decisions. He was unsure if the next poll will be implemented in the Fall, but promised “beginning of next year, at the latest.” I’m not holding my breath.

Well, the Fall of 2010 has come and gone. The EATCS council had another meeting during ICALP 2011, and Monien summarised the decision. Here’s what I was able to jot down quickly from his presentation:

• The decision is between Springer and the LIPIcs series and will be decided by the members of EATCS by a ballot “at the end of the summer.”
• The EATCS Council recommends to go with Springer.
• The poll will include the following documents: The proposals from both Springer and LIPIcs. The recommendation of the EATCS council. A dissenting opinion from a minority in the Council. A list of pros and cons.
• The final decision is made by simple majority among EATCS members. A quorum of 25% is necessary.

For a quick calculation, the report currently lists 787 EATCS members. So some 200 members would need to respond. Get out the vote!

As a sign that times are changing, the ICALP proceedings were online and digitally available to all conference attendees at the time of the conference. Splendid.

Name Tags

I obsess about name tags.

ICALP 2011 chose the neck band solution, which I derided in a brilliant piece of investigative journalism Post ALGO 2009.

In the comments of that post, G. Philip suggested to print names on both sides of the tag, so as to increase the probability of legibility close to 1. Remarkably, this was exactly what the organisers had done!

(Again, enterprising conference attendees managed to defeat the good intentions. The lunch tickets happened to fit exactly in the name tag pocket, so that with probability close to 1/2, the tag showed the name badge, and with probability close to 1/2, it showed lunch tickets. Incidentally, lunch tickets also had your name on them, but only printed on one side. Thus, in total, attendees were identifiable with probability around 3/4.)

But the big problem with neck bands is that the name badge ends up below table height during lunch and also otherwise is awkward to inspect.

The layout of the ICALP 2011 badge is fair. A good part of the real estate is actually taken up by the attendee’s name. Also, accents were correct as far as I could tell.

In fact, the organisers chose to display first names in larger size, giving the event a friendly touch. I like this. However, during a thoughtful lunch conversation, somebody (I don’t know who because his name tag was dangling below the table) mentioned that the last name is more useful for striking up conversations. Especially newcomers would prefer to know that they’re talking to Turing instead of Alan.

I’m undecided about the name size issue. Maybe another EATCS poll can decide it.

ESA 2011 is colocated with IPEC under the ALGO 2011 umbrella, so there will be plenty of exciting results that week.

• Dimitrios Thilikos. Fast sub-exponential Algorithms and Compactness in Planar Graphs.
• Gwenaël Joret, Christophe Paul, Ignasi Sau, Saket Saurabh, Stéphan Thomassé, Hitting and Harvesting Pumpkins, arXiv:1105.2704.
• Fedor Fomin, Ioan Todinca and Yngve Villanger, Exact algorithm for the maximum induced planar subgraph problem, Todinca’s slides from Worksh. Graph. Decomp. 2010.
• Marek Cygan, Marcin Pilipczuk, Michal Pilipczuk and Jakub Wojtaszczyk, Scheduling partially ordered jobs faster than 2ⁿ.

I’m probably missing some results, but without online abstracts it’s hard to tell. I cannot judge a paper from its title alone. Comments and corrections are welcome, in particular links to online manuscripts.

I cannot wait.

[PDF version]

Image source: Country Walks of a Naturalist with His Children by W. Houghton