Octave

This really is GNU Octave running on your device.  It is full featured and professionally supported. 

This lets you run Octave / Matlab code on your phone (not the cloud) and without restrictions.

About Octave:
GNU Octave supports a powerful mathematics-oriented syntax with built-in 2D/3D plotting and visualization tools.  It features a high-level programming language, primarily intended for numerical computations. Octave helps in solving linear and nonlinear problems numerically, and for performing other numerical experiments using a language that is mostly compatible with MATLAB. It may also be used as a batch-oriented language.  It has too many features to list here, but you can check out the project page for more information: https://www.gnu.org/software/octave/

How to use this Octave Android app:
If using the terminal, you just start typing commands as you see fit.
If using the graphical interface, use it just like normal.  But here are some specifics to the Android interface.
* Tap with one figure to left click.
* Move mouse by sliding around one finger.
* Pinch to zoom.
* Press and hold and then slide one finger to pan (useful when zoomed in).
* Slide two fingers up and down to scroll.
* If you want to bring up a keyboard, tap on the screen to get a set of icons to appear and then click the keyboard icon.
* If you want to do the equivalent to a right click, tap with two fingers.
* If you want to change the desktop scaling, find the service android notification and click the settings.  You have to stop and restart the app after changing this settings for it to take effect.
This is all easier to do on a tablet and with a stylus, but it can be done on a phone or using your finger as well.

To access files from the rest of Android, there are many useful links in your home directory (/home/userland) to places like your Documents, Pictures, etc.  No need to import or export files.

If you don't want to, or cannot pay the cost of this app, you can run Octave via the UserLAnd app.

Licensing:

This app is released under the GPLv3.  The source code can be found here:
https://github.com/CypherpunkArmory/octave

This app is not created by the main GNU Octave development team.  Instead it is an adaptation that allows the Linux version to run on Android.