There is a small Android app I wrote this summer to play around with Android-Studio and learn a little bit of Kotlin. It lets you enter a number and checks whether it's prime. If not, it tells you the prime factors the number is made of.
Once I had the app I tried to get it published in Google Play but you need quite a community to cover the requirements for testing so I trashed that idea and submitted the app to F-Droid where it finally got published: Is it Prime?
Source code is available in my GitLab repository.