Proof that FreeFem allows a Simple Integral, to be restated as a PDE, and then solved over 1 axis.

My recent postings have rambled at some length, about the open-source program ‘FreeFem’, the purpose of which is, to solve Partial Differential Equations, which are strictly defined, but which often won’t have exact, analytical solutions. FreeFem approximates their solutions numerically.

My own formal background doesn’t extend much beyond Calculus 2, such that I wasn’t even taught “Ordinary Differential Equations” – aka ‘ODE’s – in a classroom. But what that really means, is just, that I can’t solve one manually. I can still comprehend what problem is being defined, and, given computers that can solve those, can also feed them to my computers to solve.

Of course, ‘PDE’s are more difficult than ODEs, because PDEs are multi-variable.

Long story short, my recent postings have had two main subjects: They have asserted that real-world PDEs, like real-world ODEs, are usually more complicated, than just a form that can be converted directly into an integral. And secondly, I’ve mused over how, then, FreeFem will go about solving them anyway. That second part is speculative.

But, just to make my point, the following is a PDE, which only makes full use of 1 out of its 2 available variables, and which happens to be simple enough, to state a simple integral, but to state it implicitly. Here the FreeFem script:


// For label definition.
int Bottom=1;
int Right=2;
int Top=3;
int Left=4;

// The triangulated domain Th is on the left side of its boundary.
mesh Th = square(10, 10, [(x*2)-1., (y*2)-1.]);
plot(Th, ps="ThRectX.eps", wait = true);

// Define a function f.
func f = x * y;

// The finite element space defined over Th is called Vh here.
fespace Vh(Th, P2);
Vh u, v;	// Declare u and v as piecewise P2-continuous functions.

// Get the clock in seconds.
real cpu=clock();

// Define a PDE, that just integrates f with respect to Y...
solve SimpleXInt(u, v)
	= int2d(Th)(
		dy(u) * v			// Let this be valid syntax.
	- int2d(Th)(
	+ on(Bottom, u=0);		// The boundary condition at the start of the integral.

// Plot the result...
plot(u, cmm="Is this an example of the Poisson Equation? f=x*y",
	ps="SimpIntX.eps", value=true, wait = true);

// Display the total computational time.
cout << "CPU time = " << (clock()-cpu) << endl;


The way this script works hinges on a simple idea: ‘dx(u)‘ can accurately be computed by FreeFem as ‘the derivative of (u) with respect to (x)’, and evaluates to a real number. Since it was possible just to multiply a function that also evaluates to a real number by ‘v‘, and thus form the RHS of the equation, it should be just as easy, to write ‘dx(u) * v‘ as the LHS. And, after having fixed some minor technicalities peculiar to computing first-order integrals, one can see that this valid syntax computes ‘f(x,y):=x*y‘, and then integrates it once, in the direction that the Y-axis is positive.







(Updated 7/25/2021, 22h00… )

Continue reading Proof that FreeFem allows a Simple Integral, to be restated as a PDE, and then solved over 1 axis.

Possible Solution to ‘Dropbox Missing Systray Icon’ under KDE

Just yesterday I installed the proprietary version of “Dropbox” on my ‘Debian / Jessie’ computer named ‘Phoenix’. I had the extra HD space to spare, and also had an existing Dropbox account to link to. But what I soon noticed, was the fact that I was suffering from the same problem many other users of ‘KDE 4′ were having with the newest Dropbox for Linux.

It seems that the Linux versions of Dropbox are tuned to work best under ‘Ubuntu’, not ‘Debian’. And in General, Ubuntu uses either ‘GNOME’ or ‘Unity’ as its desktop manager, which leaves many KDE users having to use the official Command Line Interface for Dropbox.

Mind you, this CLI is not bad as those go, but missing the System Tray Icon was annoying me. I had to install ‘libappindicator…’ as well as ‘python-appindicator’ from the package manager, and even after having done that, and after having restarted Dropbox using the CLI, the systray icon did not appear for me, because in recent Dropbox versions, only the ‘Nautilus’ support is included. Nautilus is the Ubuntu / GNOME counterpart, for what ‘Dolphin’ does under KDE. Luckily, there is an open-source Dolphin plugins package named


But that package assumes we already have Dropbox installed, and does not affect the system tray.

Further, I was disappointed by the fact that most of the other complaints I could Google involved KDE 5, while I needed to solve this problem with KDE 4.

And so after doing some more reading, I wrote the following script:

(Edit 03/31/2016 : ) I would like to thank Darwin Silva, who suggested a solution below, which works better for me, than the solution which I had first posted. Specifically, the solution by Mr. Silva allows Dropbox to animate the icon correctly, to show its status. I apologize for taking so long to test Mr. Silva’s solution, but often have limited time to go after all the things I should be doing on my own computers:


# "Restart"

# Allow Dropbox 3.14.7 to show a system-tray icon.
# Works under KDE 4.14...
# Only drawback: Icon has generic page as image.
# Systray Icon Context Menu fully functional.


dropbox stop

sleep 30

dbus-launch dropbox start

This seems to have done the trick for me. But be warned: This will serve us at best, Until the Next Reboot. Possibly, this script may need to be run more often. There is a workaround which fully automates that problem, but that workaround was not worth my while.

It may be possible to edit


But doing so is pointless, because Dropbox will overwrite that file, every time it updates itself…