Analysis of Scheduling Problem on Two Machines with Partial Precedence   Order on the Set of Jobs
  In this paper, we consider an NP-hard problem of scheduling a set of jobs of equal processing time on two machines, given a partial precedence order on the set of jobs, with an objective to minimize the makespan. An approximation algorithm is proposed for this problem with a tight approximation bound. Polynomial solvability of the problem is proved in the case when each job on the first machine is in precedence relation with two jobs on the second machine. 
Anna Romanova
math.OC