Octave for Android
Termux
Termux is a Terminal Emulator for Android. You can install it from Play store or F-Droid. You don't need a rooted device to use Octave with Termux.
Direct Octave installation
You can directly install Octave in Termux using the repository by its-pointless.
wget https://its-pointless.github.io/setup-pointless-repo.sh
sh setup-pointless-repo.sh
apt install octave
However, graphics capabilities are not available with this approach. It would only plot with ASCII characters in the terminal.
Via some GNU/Linux distribution
Inside Termux in Android, you can install a Linux distribution like Debian, Ubuntu, Arch or Alpine. Note that Alpine consumes relatively less disk space. Octave can be installed as described in Octave for GNU/Linux from the distribution's respective package manager.
To use Octave's graphical capabilities, one needs to install a desktop environment, and a VNC server in the GNU/Linux distro, and one also needs to install a separate VNC viewer app in the Android phone (can be installed from Play Store/F-Droid). On many android devices, only the graphics toolkit "gnuplot" is known to work. Several apps (e.g. Anlinux, Andronix) are available to easily install the Linux distribution of your choice, and setting up the desktop environment. Refer to the Andronix docs for an overview of setting up the VNC server.
GNURoot Octave
The Android app GNURoot Octave in the Google play store is built and maintained by Corbin Champion. However, note that these repositories have not been updated for several years. It is not part of the GNU Octave project. Thus please use the following GitHub pages for questions and bug reports:
The implementation has been done in close cooperation with the Octave developers and makes use of the Octave source code without essential changes. Thus, it is fully compatible with the versions of Octave on other platforms.
Some probably outdated build instructions for the older "octave4android" app: