I have been using Math Stack Exchange for only 3 days now but am already in love with it. I have a very simple question "Why isn't there a Math Stack Exchange app?"
You can still download the official Stack Exchange iOS app from the App Store, but it's not clear for how long. There used to be an official Android app as well. However, the company stopped working on them a while ago:
We are not providing any further support for the mobile apps. There will not be any more builds made for either the Android or the iOS app (even for breaking bugs). We encourage all mobile users to use the responsive website. We hope to continue to incrementally improve the user experience for mobile users.