Which APP should I use if I want to draw a workflow with mathematical expressions on Mac? I want to find a graphic application that can draw basic workflows with mathematical expressions. For example, I want to draw something like the following figure from "Implementation and Optimization of Differentiable Neural Computers" by Carol Hsin.
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.