I came across this expression in the documentation for some software:
Here, $\alpha$ is the element of a matrix with the rather curious index "$i*(j)k$", which the documentation describes as being a mapping function from the elements of one matrix to another. I have never seen this notation before in my life, specifically the use of the multiplication operator and the parentheses. I am familiar with Einstein notation for matricies, but this does not seem to be related to that. It is possible that this is just a typo in the documentation, but in case anyone can make sense of it, please help me understand! Thanks.