Category Archives: IT

FEniCS differences between Function, TrialFunction and TestFunction

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The FEniCS project allows for simple solution of partial differential equations. However, getting started from examples is so quick, it is easy to miss how the inner-workings of it behave. This should not happen (especially if it is a PhD project). There are three vital pieces of the puzzle that it might not be clear what they are for, and the documentation does not help here: the functions TrialFunction, TestFunction, and Function.

The thing is that, in the usual Finite Element Methods, we only have the distinction between the trial function u \in V_h and test function v \in V, and even then the distinction is often blurry. In fact, technically, the spaces V_h and V can be different. However, most of the time we take them to be the same (for example, the same discrete space of piecewise linear functions). The names we give them is just to have clearer in our heads what role they play in the game, but we could as well say “two functions in V“. Then why would FEniCS have even a three-fold difference? You can quickly check for yourself that, for example, if you swap the TrialFunction for a Function, FEniCS will complain.

There is no documentation on the topic as far as I have found, so these are my (maybe mistaken) deductions.

The meaning of the TrialFunction is to let FEniCS know what function we are solving for. In fact, when we call

FEniCS expects to find a TrialFunction in A, which is supposed to be the function we are looking for. On the other hand, u_ is the symbol in which the result will be stored, and this needs to be a Function (not Trial, not Test, just a regular Function).

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Troubleshooting the installation of IRAF on Ubuntu

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So, found myself installing IRAF on a friend’s laptop running Ubuntu. There are some decent tutorials online about the general steps: for example, this one, and the official one. However, they all skip all possible issues that could come up (or at least, that popped up in my case). This is another good resource about the setup, config and usage (but skip the Ureka parts).

The community-maintained version of IRAF allows easy installation on some systems, such as Ubuntu.

The thing is that IRAF is a jumble of stunningly old pieces of software working together on primitive terminals and on peculiar conditions.

In random order, possible issues/tips are:

  • If the packages iraf-all pyraf-all stsci show as non-existent, you have not added the astroconda channel. The command should be
    conda config --add channels http://ssb.stsci.edu/astroconda
  • You will still need to manually install ds9:
    sudo apt install saods9
  • You do not need to change the default shell to tcsh, although you need it to be installed:
    sudo apt install tcsh
  • If xgterm does not execute with error File not found, although the file is clearly there, make sure you have installed 32 bit dependencies:
    sudo apt-get install libc6:i386 libz1:i386 libncurses5:i386 libbz2-1.0:i386 libuuid1:i386 libxcb1:i386 libxmu6:i386
  • If conda commands do not work, make sure you have activated the Python environment containing the iraf packages:
    cd /path/to/iraf
    source activate iraf27

In the end, the exact set of commands that got the whole environment working has been: Continue reading →

Install Windows 7 on a modern, Intel 8th generation computer

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I spent a good week trying to get Windows 7 to work on a modern laptop, having an Intel i3 8th generation CPU and other recent components, such as a NVMe SSD. What I did not know in the first place, is that officially, Windows 7 does not support Intel CPUs later than 6th gen. Moreover, it does not support UEFI boot (not GPT partition tables). For reference, we are talking of a PC Specialist Ultranote IV 14″. Up to now, everything works almost flawlessly but the wifi/bluetooth adapter.

I am not gonna provide a full tutorial on how to install Windows 7 on a modern computer, but I am going to list here all the relevant resources that helped me succeed in the mission, as the research turned out to be quite exhausting.

The main issue is that the installer will stop quite soon and abruptly, stating that “A required CD/DVD drive device driver is missing“. Clearly, this is bullshit, as the laptop does not even have an optical drive and the installer was running from USB.

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Endless Christmas X-MAS CTF Writeup

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This is a writeup for the Endless Christmas challenge, md5 hash 866c92038d6e9fc47db4424f71f6167a (download binary). It appeared in the X-MAS CTF, and it’s a Reverse challenge.

Using afl with Radare we can see there are calls to write and execve, both happening in main, a sign that this program creates (and maybe executes?) something else.

Putting a breakpoint just before the execve happens will reveal what file is being loaded (looking into the rax register).

I went down 60 rabbit holes disassembling this binary further, but the best thing we can do at this point is change point of view, step out of Radare, and launch the binary by itself – it certainly doesn’t seem to be doing anything nasty up to this point.

It takes some time before any output is shown, so this may be a sign that some decoding happens. The program creates a good number of other binaries which all look identical, albeit different from the original one (as their size shows), but that are actually different upon closer inspection with their md5 hashes.

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On knowing when to stop in software development

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One of the very few things I learnt in art class is what the role of Jackson Pollock was in his art. Because, we were asked, what is the role of the artist, if the only thing he does is let paint drop on a canvas? His role is to decide when the work is complete.

This is something we most often overlook in computer science: there comes a time when a project, or a feature, is complete, and any more improvements, any more work put into it is likely to decrease its value and ruin all the good work. Too often we want progress in our applications, without realizing that it’s actually destroying them. Sometimes it’s just better to move on and work on something else. Even if a solution is 10 years old, it doesn’t mean it has to be updated because progress requires it.

Let me present a couple examples.

The Gutenberg editor in WordPress

WordPress 5 introduced the new Gutenberg editor, a project that has been rated with 2 stars out of 5 with a total of around 2000 reviews at the time of writing. It’s a product that is so buggy and un-usable that it is bewildering that it made it into Core, but whatever (in 10 minutes of usage, I found 7 crucial and unreported bugs just 4 months prior to release – see my review).

gutenberg reviewsLet us pause and ponder why it was introduced. Any apology of Gutenberg will say that is because the classic editor felt old. It looked so much like Office 2003, and it’s 2018, they say! You see, they say, 15 years in computer science is a huge deal!

But, you see, what is the main purpose of an editor? To write. And to that it must be apt. Gutenberg shifted the focus from writing content to designing a page, effectively forcing a progress in the wrong direction. Not much has changed in writing since Office 2003 came around: we still use bold, italic, headings, text alignment and little more. Anything else requires the careful crafting of a designer and writing of some HTML, as it should be. Nothing else is needed, really, when it comes to writing. But, they say, you cannot even create a table with the classic editor! And I say, that’s right, it should be possible! But that doesn’t require trashing a whole editor and building a cumbersome React-y thing just so that we can have tables, does it?

But, they say, this way you don’t need a designer to design your pages anymore. Of course, people must be really stupid if they have been paying web-developers/designers to put up their websites for the last 25 years, of course! So stupid of us! There, instead of hiring a professional photographer to shoot at your weeding, just give a compact camera to your uncle, since technology and progress have enabled you to do so. Because it really is just the same. When I was a kid, websites designed with Dreamweaver were looked down on, and anybody who wanted a real site should have hired a professional. Not it looks like everybody can do everything – expect that, uhm, they can’t.

Too often the right questions are not asked and carefully considered. Those are the most basic ones: do we really need this thing? How difficult is it to build it? Is it really worth it? What is the impact it will have on users/market? Does it add something really useful and needed without breaking anything else?

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