I just got a new phone, and realized that the only game I occasionally play, MathDoku, won't run anymore!
This was unacceptable, so I wrote my own: https://magnusm.uber.space/mathdoku/
Github: https://github.com/magnusmanske/mathdoku (GPL, feel free to run it on your own server if you prefer)
This is a "progressive webapp", you can play it in the browser, or install it like a "proper" app on your mobile (Chrome/Android trhee-button-menu, "Install app"; other borwsers/platforms, please let me know!). It will keep your game stored even during reboots.
Issues and merge requests welcome!
Seconded this. Very small too, at about 10 MiB, or 250 kiB per game, which is tough to beat! The Android port is probably the best but a Linux and Windows version also exist. Get it on F-Droid
There is also Gauguin, a similarly small app but with only 1 puzzle type, this one. There are more options though, including more operation combos, negative and non-consecutive numbers, and even rectangular grids! The controls are a little worse if you ask me and so is the performance but not by a lot. I have spent huge amounts of time playing the game in both apps and I slightly prefer this one thanks to more variation.