Sometimes, it can be difficult to find the mathematical symbol you need. There are several options here. The first (and quickest) is to use Detexify, where you draw the symbol you'd like, and it tries to find what you want, like as shown below:
Another option is to use the comprehensive LaTeX symbols list, which can be found here. If you are using the package
unicode-math this list of all supported symbols can be helpful. Another option is this website, which has common math symbols.