summarize this:
The resource constrained project scheduling problem with a flexible project structure and consumption
and production of resources, involves making a selection of activities and scheduling these activities in
order to minimize the makespan, subject to precedence and resource constraints. Since finding a feasible selection of activities is NP-hard, we introduce the concept of group graphs and restrict ourselves
to instances with an acyclic group graph. For these instances, which represent many practical cases, we
show how to make a feasible selection of activities in polynomial time and use this concept to schedule
the selected activities using a hybrid differential evolution algorithm. We compare this algorithm with
an algorithm from the literature on special cases of instances without consumption and production of resources, and show that our algorithm creates solutions of higher quality. Furthermore, to compare general
instances, we develop an ant colony optimization algorithm that performs slightly better on special cases
than the algorithm from literature and show that the hybrid differential evolution algorithm outperforms
the ant colony optimization algorithm on general instances