I have now also succeeded in installing xmaxima, an open-source alternative to Mathematica and Maple, under Easy Debian. That part actually is completely trivial: just "apt-get install xmaxima".

Then it took me a while to find out why I couldn't have my input evaluated. Pressing the return key just did nothing. Finally I found out that under LXDE the return key had the value KP_Enter instead of Return, which for the other applications didn't seem to make a difference. Fixing that with solved the problem. I then used xmodmap also to have square brackets and other special characters on the hardware keyboard, and then everything worked.

I certainly prefer doing symbolic math on a laptop computer since it's quite tedious to type complicated expressions on this tiny keyboard. But the program itself runs very smoothly - one can even rotate those 3D plots with the cursor (=stylo)!