Big Table of Binary Fluorine Compounds

While not as common as for instance oxygen, the element fluorine (F) reacts and forms compounds with almost all other elements.  In fact, fluorine is the most electronegative element (except for the unreactive neon on one particular electronegativity scale), and this gives it particular properties:  The pure element, the gas F₂, is extremely reactive, so much that it will react with almost all materials except a few noble gases (such as neon), certain other unreactive substance (such as nitrogen at standard conditions), and compounds that already contain fluorine (such as teflon).  In fact, the history of fluorine is highly interesting in itself, having claimed the death of more than one chemist who tried to isolate the pure element.

The following table gives an overview of the binary compounds of fluorine.  It shows compounds that contains the element fluorine (F) and another element.  The compounds are arranged by oxidation state and stoichiometry, showing that many elements form multiple compounds with fluorine.  Since fluorine is more electronegative than all elements with which it forms binary compounds, all compounds are called “fluorides”, the exception being fluorine azide (FN₃).


Download PDF

  • The table contains links to all Wikipedia articles, or to research papers if the Wikipedia article does not exist yet
  • The compounds are coloured according to their type:  molecular compounds in blue, ionic compounds in black, etc.  The exact legend is given within the plot.
  • As a bonus, we also include the binary fluorine minerals.

Please post additions and corrections here.

UPDATE:  Version 3

How to Pronounce German Vowels

The German language may be known for its many consonants, but what most learners have trouble with are in fact the vowels.

The following chart gives a pretty much thorough account of the pronunciation of German vowels, in form of a flowchart.  It will help you determine how to pronounce the vowels in a given word.


Download chart as PDF

UPDATE:  This is version 5 of the chart, as of 2017-02-17.

Please comment here with suggestions, corrections, etc.

A Mathematical Riddle

I’ll just place this here:


You have to compute this expression.

This riddle was composed a few months ago for a competition, but we decided to not use it in the end.  Actually, it is easier than it seems:  You don’t need any calculator or computer to solve it.  A piece of paper (or two) should be enough.  We wrote the riddle in a way that there are several shortcuts that can be used to solve it, but if you don’t find the shortcuts, you can still solve it in the obvious way, taking just a little bit longer.

Also, you have to find the exact answer, not a numerical approximation.

I’m not posting the answer here for now.  If you’re curious whether your answer is correct, post below.


No Hairball – The Graph Drawing Experiment


Many graph drawings look like a hairball.

The larger a network is, the harder it is to visualize it.  Most graph drawing algorithms produce a giant “hairball”, in which nodes and edges are hopelessly mixed up, leaving no way to discern any structure whatsoever. Here is an example:


This is from one of my own papers (WWW 2009), so I should know what I am talking about. Nowadays, I wouldn’t put such a picture in paper, let alone on the first page as I did then.  Many papers however, still contain such graphics.

Can we learn anything from this drawing?  No.  There are no communities visible.  No clustering is apparent.  I cannot even tell whether the graph is bipartite.  In fact, I cannot even estimate the size of the graph from this picture.

So, why do we keep putting hairballs in our papers?  Maybe, because they give us the illusion of insight into a complex network.  Yes, we would like to understand whether a graph displays clustering, bipartivity, assortativity, dissortativity, skewed degree distributions, and a myriad of other interesting properties that complex networks can have. What better way to visualise these features, than by drawing the graph?  Isn’t every visualisation also a drawing?

No, visualisations are not necessarily drawings.  We don’t need to draw a graph in order to visualise it.  In fact, the mere fact that we try to draw the entirety of a network in a small space is what leads to the hairball problem to begin with: Hundred of thousands, millions or even more nodes and edges are crammed into the space of only a few centimetres.  It is no wonder we don’t see anything in a hairball graph:  Each node and each edge gets allocated a space on the order of a micrometre or less – much too small to even be shown on computer displays or printed on paper, let alone seen by the human eye.

What then, can we do to avoid hairballs?  Several ideas have been tried:

  • Show only a subset of the network.  This is called sampling.  In its simplest incarnation, choose a random subset of the nodes, to get a subgraph of the real graph of manageable size.  Then, draw that subgraph.
  • Aggregate nearby nodes into single nodes.  This is called coalescing, and also produces smaller graphs, which are then drawn with the usual methods.
  • Allow the user to zoom in, using interactive software.  This is nice, but doesn’t give any more insight into the overall properties of the network.

These methods are all suited to particular use cases.  But for visualising overall properties of a graph, these methods fail.  What do these methods all have in common?  They assume that to visualise a graph, you have to draw the graph.  The question thus becomes, Can we go further?  Can we visualise a graph without drawing it?  Almost.

In the experiment we are performing, we don’t draw the graph to be visualised.  Instead, we draw another graph – a much smaller one – which is representative of our graph.  In fact, we throw away all nodes and edges of the input.  Only graph statistics such as the assortativity, the bipartivity, and others are kept, and a completely new graph is created, specifically for visualisation.  Most graph/network researchers now think, How can I see individual nodes and edges of the graph?  The answer is that you can’t; that is the point of the method.  We sacrifice local information in graph in order to make global properties more apparent.

The method is in development, and a paper is upcoming, but not public yet.

We are now performing an experiment to find out how to do this graph visualisation optimally:


This is an interactive experiment that will ask you to look at graphs, and to answer yes/no questions. In fact, you only have to click on graphs to answer.  However:  You must be knowledgeable in graphs and networks to participate.

This will help our research.  If you want to keep in touch with the results of the experiments, write to <>.

If you’re interested in graph visualisation, check out the KONECT project; it has lots of graph visualisation methods based on matrix decompositions.


Which Elements are Named After Countries?

This week, names of four newly discovered chemical elements have been announced:

  • Nihonium (Nh) for element 113
  • Moscovium (Mc) for element 115
  • Tennessine (Ts) for element 117
  • Oganesson (Og) for element 118

These names are up for being revised in six months. Barring some unexpected development, these names will go down in the chemistry books – and more to the point, nuclear physics book – as the names of the elements that complete Period 7 of the periodic table.

Of these four, one name refers to a country (Japan), one to a region (Tennessee), one to a city (Moscow), and one to a person (Yuri Oganessian). All four names were chosen by the discoverers, and honour the discoverers. While the discovering teams have indeed produced a tremendous work and certainly deserve the honour, I find that type of naming quite unfortunate, and even downright selfish.

This has not always been the case. Most elements were not named for selfish reasons. Some elements were named for their properties, such as hydrogen, oxygen, and nitrogen. Some for materials from which they were extracted: beryllium from beryl, aluminium from alum, calcium from calx, sodium from soda, potassium from potash, boron from borax, silicon from silex, and lithium from stones.  Many were named for the colours of their compounds: chlorine is green, bismuth is white, rubidium is red, caesium is blue-grey, gold is yellow, indium is indigo, iodine is violet, rhodium is pink, thallium is green, chromium is colourful, and iridium has the colour of the rainbow. I find these highly poetical. Not to be outdone, some elements were named after astronomical objects: uranium, neptunium, and plutonium for Uranus, Neptune, and Pluto; cerium after Ceres; selenium after the Moon; and tellurium after the Earth.

In more modern times, many radioactive elements were named for their radioactivity: actinium and radium produce rays, astatine is unstable, technetium is artificial. Others were named for various other properties: bromine and osmium smell, argon is inactive, barium is heavy, neon is new, xenon is foreign, phosphorus carries light, krypton and lanthanum are hidden, and dysprosium is hard to get.

Newer elements however, are not named for their properties, but are simply given a name meant to honour the discoverers.  All non-black cells on the follow periodic table are elements whose names can be traced back to a country:


Periodic table of elements by country referred to in the element name.

The last row, Period 7, is almost completely coloured.

I have made some simplifications in the table:  I count Rutherford as British (by his career), and the Rhine as German. Also, the Curies are Polish here.

Let’s look at the country-based names:  Some are quite innocent, like magnesium and manganese, named indirectly after a Greek region called Magnesia. Copper gets its name from the Island of Cyprus, while Beryllium and Strontium get their names indirectly from places in India and Scotland.

In other cases, chemists have tried to honour their country, such as France (gallium and francium), Germany (germanium), and Russia (ruthenium). Note how nicely gallium and germanium are placed side by side. Also note that while europium is named for a deity, the nearby americium was named consciously after the United States.

The Scandinavian countries have the largest share of elements named after them. Famously, a whopping four elements are named after the tiny Swedish village of Ytterby: ytterbium, yttrium, erbium, and terbium. Scandium is named for Scandinavia, thulium for a mythical Scandinavian region.  Holmium and hafnium are named for Stockholm and Copenhagen, respectively. Nobelium and Bohrium and named after a Norvegian and a Danish person, respectively.

Since World War II, newly discovered elements are named simply after people, or after the countries or regions of discovery. In fact, there has been a dispute between American and Soviet physicists about naming priorities. As a result, we don’t have an element named kurchatovium, even if I learned about it in school. With the four new names, that particular dispute should be considered as settled at a score of 7–7, although I’m sure some people on either side of the debate still hope for more.

While names such as einsteinium, fermium, copernicium, and meitnerium  seem fair, given that the persons were already dead and unrelated to the particular discoveries, it seems to me quite preposterous to give names after living people who are involved in the research.  This was the case for seaborgium and the newly named (and not yet finalised) oganesson. Will all respect for these researchers, I doubt that Glenn T. Seaborg and Yuri Oganessian will go down in history in the same way as Copernicus and Mendeleev. Or Einstein and Meitner. Or Curie and Rutherford.

Also, it seems that the responsable discoverers are not trying anymore to follow latin nomenclature:  “nihonium” deserves to be called japonium. That would also give the much more memorable symbol Jp, coinciding with Japan’s country code. If I were responsible for bettering Japan’s standing in the world, I would lobby for “Jp”. On the other hand, Moscovium sounds very appropriate to me, and I like that tennessine and oganesson use the relevant suffixes for halides and noble gases, even if I doubt their corresponding chemistry is going to be characterised anytime soon.

It seems that the last element named after a concept was plutonium. After the last planet to be discovered at the time, and also after a god. The current research teams would do well to come back to naming new elements after actual properties of the material or at least to find metaphors relating to planets and deities, and not just as a vehicle for advertising their labs and principle investigators. Let’s hope that if elements in Group 8 can be synthesized, we will go back to a more classical naming culture.

Index of Complex Networks – The “Google for Networks” by University of Colorado Boulder

This week at the Conference on Network Science (NetSci 2016), Aaron Clauset from the University of Colorado Boulder unveiled the Index of Complex Networks, assembling information about thousands of network datasets available online. They have about 3500 entries at the moment, with many more to come in the near future.

This news is a big deal in the network science community. Researchers in the field of Network Science are (quite obviously) using network datasets on a daily basis, and anything that helps them get their hands on more of them is good news. Until now, there have been several sites collecting network datasets, such as SNAP from Jure Leskovec’s group at Stanford and KONECT written by me in at the University in Koblenz.

The new ICON website takes the community one step further in making available not the datasets themselves, but an index of them. I.e., a listing of datasets available on the web, whose largest share comes from KONECT, but also from SNAP and countless other network dataset-publishing sources.

What does this mean for the field of Network Science? It means we will be able to verify our claims not on individual datasets, or on a few dozens ones, but on thousands of them. This is worth spelling out: With thousands of datasets, we can finally investigate the statistical significance of claims. We may finally find out whether social networks are really scale-free. We may finally find out, whether social networks really have so many more triangles than other networks, as claimed traditionally in social network studies. In fact, we already have first results based on the ICON data: Networks as a whole are not scale-free. In other words, degree distributions are not power laws in a statistical sense. This contradicts many claims made in the field.

I expect this data to lead to many new insights. In particular, many “well-known” facts about networks will have to be revised. I have to say, I’m very much looking forward to start this new chapter of Network Science.

The Index of Complex Networks (ICON) is here:

KONECT (The Koblenz Network Collection) is here:

Stu – Build Automation for Data Mining Projects

Stu is a build tool developed at the University of Koblenz-Landau. In this article, I will explain what sets it apart from the myriad of other build tools, why I wrote it, and how we use it in various projects at the Institute for Web Science and Technologies.

What is Make?

Everyone has probably already heard of Make, a standard Unix tool mainly used in software projects for automatizing the build process. Big software projects consist of many source files, are compiled to many individual object files, and then linked. Larger software projects may even have more complex structure, for instance, they might autogenerate code, compile multiple versions of the software, or even have different compilation procedures depending on the platform on which they are run. As a result, building software is not an easy task, and many tools exist to make it easier. The quintessential tool in this category is undeniably Make, dating back to the 1970s as part of Unix. By now, Make has been standardized as part of the POSIX specification, and is ubiquitous. The best known implementation by now is probably GNU Make, as it is the default implementation on Linux.

Running Data Mining Projects

While everyone knows Make as a tool for building software, not everyone knows that it is also a completely generic build tool: It can be used to generate datasets, convert files, compile Latex documents, etc. For this reason, Make has been the tool I myself have used for years whenever I implemented data mining projects. I use the term “data mining project” in a broad sense here: any project in which data is manipulated. For instance, a typical data mining project may include the following tasks:

  • Download a dataset from the web
  • Uncompress it
  • Clean up the dataset
  • Convert to another format
  • Visualize the dataset
  • Compute features
  • Run a regression
  • Visualize the result of the regression

Most people who do data mining projects would probably write a single program for each step, in their favourite or in the most appropriate programming language. Then, they would run these programs sequentially. If necessary, earlier steps can then be re-run by hand. While this is an acceptable way to go about things in data mining projects, it still leads to many undesirable situations. Sometimes, we find out that our dataset contains individual data points that were wrongly extracted. In this case, we should adapt out clean-up code, and re-run all steps from that point on. Unfortunately, many data mining researchers do not remember the exact sequence of steps taken, and thus they simply do not perform the additional clean-up. A few erroneous data points will not spoil the end results, or so they think.

Common Problems Encountered in Data Mining Projects

In fact, the inability to re-execute previously performed steps is a very common phenomenon, and leads to many unfortunate situations: data clean-up is not performed as it should, tests are not re-run properly, plots are not re-generated, etc. All these problems can be traced back to one root: The inability to remember which steps were actually taken in the past. It thus comes as no surprise that people, as I did, turn to tools like Make to drive their data mining projects. This is a perfectly good solution to a common problem, and thus we may see code like the following:

data-clean.txt:  data.txt cleanup
        ./cleanup <data.txt >data-clean.txt

This is a snippet from a hypothetical data mining project in which Make is used. In this example, a script “cleanup” is used to clean up the dataset, reading the data from the file “data.txt” and writing the cleaned up data into the file “data-clean.txt”. The snippet can be saved in a file “Makefile”, and then “make” can be executed on the command line. Make will then execute the given command, but only if necessary. In fact, Make is smart enough to check the timestamps of the given input and output files, and only rebuild the output file when the input files are newer. This feature of Make is very useful when many rules are combined. Changing a single file and running Make will then only rebuild what has to be rebuilt.

When Make Is Not Enough

This usage of Make shows that Make is not only a source code compilation tool, but a generic build tool. The question then is, why is Make not enough? To give an answer, consider the following:

  • I need to run the same experiment on multiple datasets
  • I need to run multiple experiments on all datasets
  • I need to run the analysis using multiple subsets of all features
  • I may or may not filter out a subset of the data
  • I need to use multiple error measures

What do all these requirements have in common? The need to run things multiple times, parametrized by a variable. That variable can be the dataset chosen, the error measure chosen, or just a binary value denoting whether a certain extra is to be performed or not. Most data mining projects go about this by just implementing loops in whatever programming language they are using, and iterate over all possible values of the variables. For instance, we can easily write a shell script to clean-up all datasets in our collection:

for language in en de fr pt es ja pl nl ; do 
        ./cleanup <data.$language.txt >data-clean.$language.txt 

This is a shell script that iterates over eight languages, and calls the clean-up program “cleanup” on each. This works well as long as clean-up is fast. If the datasets in question are Wikipedia datasets for instance, then the files are likely to be huge, and clean-up slow. It then follows that if our script is interrupted, then it can only be restarted from the beginning, and will re-do the clean-up for all languages, even those where it has already been performed.

The solution to this problem is to declare each language’s clean-up in Make. For instance, we may write:

data-clean.en.txt:  data.en.txt cleanup
        ./cleanup <data.en.txt >data-clean.en.txt cleanup
        ./cleanup < >

and so on for all eight languages.

This is clearly not scalable. Instead we may use GNU Make’s feature of pattern rules, which allows us to write:

data-clean.%.txt:  data.%.txt cleanup
        ./cleanup <data.$*.txt >data-clean.$*.txt

This is much clearer, but still leaves something to be desired: How do we go about having multiple parameters, e.g. the language and the subset of features? This problem can be achieved by generating the Makefile code automatically. As we have multiple parameters, we then have to generate code for all possible combinations, leading to very large generated files, and thus a long startup time for Make, who has to parse it all. Alternatively, we can use the following type of code. This example is a real-world example lifted from the KONECT project, from before it switched to Stu:

define TEMPLATE_decomposition_all
decomposition_full.$(1).all: $(foreach NETWORK, $(NETWORKS), \
diagonality.$(1).all: $(foreach NETWORK, $(NETWORKS), \

decomposition_time.full.$(1).all: $(foreach NETWORK, $(NETWORKS_TIME), \
$(foreach NETWORK, $(NETWORKS_TIME), \
   decomposition_time.full.$(1).$(NETWORK)): \
   decomposition_time.full.$(1).%: \
decomposition_time.split.$(1).all: $(foreach NETWORK, $(NETWORKS_TIME), \
$(foreach NETWORK, $(NETWORKS_TIME), \
   decomposition_time.split.$(1).$(NETWORK)): \
   decomposition_time.split.$(1).%: \
   $(eval $(call TEMPLATE_decomposition_all,$(DECOMPOSITION))))

I am not going to explain this code. Suffice it to say that writing such code for all cases is not the way to go. (If you want to know, this is just a small snippet from the code that generates decompositions of characteristic graph matrices of networks in KONECT. Some parameters appear as GNU Make’s %, some as function parameters $(1), etc.)

A Tool That Allows Parameters in Rules

From the inability of Make to properly handle multiply-parametrized rules came my search for a tool that is able to do it. Fortunately, the landscape of build tools is very large. Unfortunately, virtually all build tools have radically different requirements:

  • Almost all are made for compiling software, not executing arbitrary commands. The few that allow arbitrary commands, such as Make, fail to allow parametrized dependencies.
  • Many are specific to a particular programming language or environment, assuming that everything will be done with it. This fails to account for the fact that any data mining project of even modest size will need to execute individual steps in multiple programming languages.

At the end of my year-long search, only the tool Cook came close to fulfilling all requirements, even though here also using some unintended hacks.

The curious reader may consult Wikipedia’s List of build automation software to get an overview of the field.

Only Two Features

Not finding any satisfying build tool, it occured to me that I only needed two features in addition to Make’s feature set:

  • An arbitrary number of named parameters in rules
  • The ability to generate the list of dependencies dynamically

Thus, I set out to write a build tool having these two features. In what later became Stu, the data clean-up example would be written in the following way:

@all:  [dep];

        echo >LANGS en de fr pt es ja pl nl 

dep: LANGS { 
        for lang in $(cat LANGS) ; do 
                echo data-clean.$lang.txt 
        done >dep 

data-clean.$lang.txt:  data.$lang.txt cleanup {
        ./cleanup <data.$lang.txt >data-clean.$lang.txt

This snippet of Stu code shows the following two features:

  • Parameters are written using ‘$’, like shell variables. They have the same syntax within filenames and in the shell command.
  • The brackets ‘[…]’ denote that dependencies are read from the content of the given file. Stu will first make sure the file is generated, and then parse the file.

These two features are the essence of Stu. They set it apart from other build tools, and are the reason I wrote it.

Other Features

In fact, the example in the previous section also shows another very convenient idiom used very often in Stu files: The list of languages is itself a file, instead of variable as it would be in Make. This has the advantage that we can add a new language and all necessary steps will be executed automatically. Also, variables in Make are usually a scalability problem: They are regenerated at each invocation of Make. Even though the main purpose of Make is to avoid unnecessary executions, any variable (whose content often would depend on other variables such as the list of languages) is regenerated many times.

In fact, Stu initally also had variables like Make, but I removed the feature once I noticed that they are unnecessary as files are always better suited for what they do. As a matter of fact, I sometimes think of Stu as a programming language in which the variables are files.

(Note: That was in Version 0.  Now, in version 1.9, features are not removed anymore, to keep backward compatibility.)

Other Facts About Stu

Like many other Make replacements, Stu corrects the typical shortcomings of Make:

  • Error messages are much better (Make has the dreaded “missing separator”); Stu gives full traces like a proper compiler.
  • Stu has a proper tokenisation pass, instead of Make’s variable substitution syntax, avoiding many quoting and escaping problems.
  • Stu catches typical Makefile errors such as dependencies that where not built.
  • Stu has better support for interrupting large builds (Make will often hang or leave processes running).
  • Stu avoids the “inner platform” antipattern present in Make, in which a lot of shell functionality is duplicated in Make functions. Stu encourages all program logic to be implemented in rules, i.e. using a proper shell.
  • Stu supports additional types of dependencies (existence-only dependencies with the prefix ‘!’, optional dependencies with ‘?’, and trivial dependencies with ‘&’). These can only be emulated partially with Make by using unwieldy constructs.

Other design considerations of Stu include:

  • As a use case, focus on data mining instead of software compilation. There are no built-in rules for compilation or other specific applications. Allow use case-specific rules to be written in the language itself. However, Stu is use case-agnostic.
  • Files are the central datatype. Everything is a file. Think of Stu as “a declarative programming language in which all variables are files.” For instance, Stu has no variables like Make; instead, files are used.
  • Files are sacred: Never make the user delete files in order to rebuild things.
  • Do one thing well: We don’t include features such as file compression that can be achieved by other tools from within the shell commands.
  • Embrace POSIX as an underlying standard. Use the Bourne shell as the underlying command interpreter. Don’t try to create a purportedly portable layer on top of it, as POSIX already is a portability layer. Also, don’t try to create a new portable language for executing commands, as /bin/sh already is one.
  • Keep it simple: Don’t use fancy libraries or hip programming languages. Stu is written in plain C++11 with only standard libraries.
  • Have extensive unit test coverage. All published versions pass 100% of unit tests. Stu has 500+ unit tests. All features and error paths are unit tested.
  • Stability of the interface: We follow Semantic Versioning ( in order to provide stable syntax and semantics. Stu files written now will still work in the future.

Using Stu In Your Project

Instead of having a “Makefile”, have a “main.stu” file. Instead of calling “make”, call “stu”.

To see an example of a large and complex Stu file, see the Stu file of the KONECT project.

Documentation is given in Stu’s manpage.

To use Stu, get it on GitHub and compile it yourself. Stu is written in standard C++11.

Stu has an extensive set of unit tests, and is quite stable. The specification of Stu strictly follows Semantic Versioning, and therefore Stu files will remain compatible in the future.

The name “Stu” follows the precedents of Make replacements Cook and Bake in referring to kitchen-related verbs, and also honours the author of the original Unix Make, Stuart Feldman.

Stu is free software, placed under the GNU General Public License, Version 3.

Stu was written by Jérôme Kunegis.