Which APP should I use if I want to draw a workflow with mathematical expressions on Mac?
Keynote on the mac can do both. It has standard shapes and the ability to draw lines connected to those shapes.  It's a simple way to draw a flowchart. If also supports mathematical expression thru latex. There is an option to insert an equation and you can define anything you like by using latex.

Here is a link to help you with Latex. On this link, you can see what the latex command is for most any symbols you can imagine. I use this page to help me find the correct latex command to put in Keynote. 

I have included a very simple and crude flowchart I made in about 5 minutes using Keynote.  The math equations are bogus as I was just trying to show the capabilities of Latex.