This lower bound is computed in a very peculiar way. Let's refer to the same picture and we define one subset of nodes here (yellow) and another operation here (red) and define the yellow set to be the critical set J and use this function which takes the minimum of the release dates for the set J plus all processing time for J and the minimum over all q_j for the same set J. This is a very effective lower bound which leads to a good branch-and-bound procedure to solve optimally the one machine problem.