Constraints of availability in timetabling and scheduling

The basic class-teacher timetabling problem is examined with the additional constraints due to the (un-) availability of source teachers and/or classes at some periods. We mention a generalization of this problem which occurs in image reconstruction problems in tomography. Complexity issues are discussed for both types of problems and some solvable cases are presented which can be derived from the image reconstruction formulation. Reductions to canonical forms are also described. Some other types of unavailability constraints (for classrooms or for lectures) are also reviewed.