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 https://f-droid.org/en/packages/org.piepmeyer.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.