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I'm studying the paper "Minimizing Total Tardiness on a Single Machine Using Ant Colony Optimization" which has proposed to use Ant colony optimization to SMTWTP.

According to this paper:

Each artificial ant iteratively and independently decides which job to append to the sub-sequence generated so far until all jobs are scheduled, Each ant generates a complete solution by selecting a job $j$ to be on the $i$-th position of the sequence. This selection process is influenced through problem-specific heuristic information called visibility and denoted by $\eta_{ij}$ as well as pheromone trails denoted by $\tau_{ij}$. The former is an indicator of how good the choice of that job seems to be and the latter indicates how good the choice of that job was in former runs. Both matrices are only two dimensional as a consequence of the reduction in complexity

They have proposed this formula for the probability that job $j$ be selected to be processed on position $i$:

but I can't understand what $[]$ surrounding $\eta_{ij}$ and $\tau_{ij}$ indicates. Does it show that these values are matrices?

1By the way, in the past, I've implemented the ant colony system (ACS). I've looked at my implementation and it seems to be consistent with my explanations above. – nbro – 2019-11-01T16:38:47.700