I came across this expression in the documentation for some software:
$\displaystyle\sum\limits_{j=1}^c\alpha_{i*(j)k}q_j=Q_k$
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.
As far as I can tell, this obscure notation was invented by the authors of the paper. In short, there are two vectors $I$ and $J$, and by definition $J$ is a subset of $I$. Therefore, any element $J_j$ has a corresponding element $I_i$, and the mapping function between these two elements is denoted by "$i*(j)$". In practice, this is used to say that when the index $j$ is used (such as in the summation in my original post), we can instead map it to a corresponding index $i$ and pull the value of interest from $I$ instead of $J$.
The paper in which I found this is publically available if anyone cares to read it. It is technical report number AD/A-002 791 by Stanford Research Institute, from March 1974. The title is "Tiger Computer Program Documentation". A Google search for the title should give you a link to the report as hosted on www.dtic.mil. The equation of interest is on page I-B-4, second full paragraph.
Thanks to all who helped.