I read many documents about ant colony optimization but I didn't understand well the process of pheromone update. I know that in the beginning, all the paths have the same pheromone trail. I want to know if, after an iteration, the pheromone will be updated only on the used path or this value will be updated on all paths (I mean if the pheromone trail on the unused path will be equal to (1-r) tau0, where r is the evaporation rate and tau0 is the initial pheromone trail)? thank you in advance
Pheromone will be updated only on the used paths. However evaporation will be on all paths.
tau0 = (1-r) tau0 + (new_tau)* k, where k is the number of ants using the path.