reading a sociology paper (https://link.springer.com/article/10.1007/s11577-015-0343-4, german) for a course today, I stumbled upon the following equation:
It's supposed to describe a 2-level "random intercept" (intercept-as-outcome) regression model where j denotes a level 2 group and i denotes level 1 case within a group. I take the sums to sum over the number of variables on each level respectively. Obviously terms 1, 3 und 5 stem from the level 2 equation. What I for the life of me can't figure out is the fourth term. Is that supposed to be an interaction? Are there two sum signs missing before it or where do the ks come from? Why is there a product of coefficients? What am I missing here?