Also known as the tensor product of linear maps in a particular basis:<p><a href="https://jeremykun.com/2014/01/17/how-to-conquer-tensorphobia/" rel="nofollow">https://jeremykun.com/2014/01/17/how-to-conquer-tensorphobia...</a><p><a href="http://www.math.harvard.edu/archive/25b_spring_05/tensor.pdf" rel="nofollow">http://www.math.harvard.edu/archive/25b_spring_05/tensor.pdf</a><p><a href="https://www.dpmms.cam.ac.uk/~wtg10/tensors3.html" rel="nofollow">https://www.dpmms.cam.ac.uk/~wtg10/tensors3.html</a>