The Matrix Cheatsheet by Sebastian Raschka.
Sebastian has created a spreadsheet of thirty (30) matrix tasks and compares the code for each in: MATLAB/Octave, Python NumPy, R, and Julia.
Given the prevalence of matrices in so many data science tasks, this can’t help but be useful.
A bit longer treatment can be found at: The Matrix Cookbook.
I first saw this in a tweet by Yhat, Inc.