Approach0 is a math-aware search engine. “Math-aware” means you can add math expression(s) as some of your keywords to have search engine help you find similar expressions. Check out here: https://approach0.xyz
To restrict matches to this site paste the string "AND site:math.stackexchange.com" into the search window (this removes AoPS matches - very useful when searching for duplicate targets)
This is my side-project, hopefully it can be useful in some cases to help Math SE users find more similar questions before posting a question for help.
I would love to hear any feedback from you guys.
EDIT: If you are unable to get expected results or have questions related to searching on Approach0, Please go to this chat room (otherwise there will be too many comments in this thread). For those who want to report a bug (not have to be confirmed) or request a feature to Approach0, please go to project issue page. (You do not have to be familiar to GitHub to create an issue, just click that button and write some description on things you feel not running well). This will attract potential contributors from GitHub to participate and get developers' attention.