Minimizing ROBDD sizes of incompletely specified Boolean functions by exploiting strong symmetries

We present a method computing a minimum sized partition of the variables of an incompletely specified Boolean function into symmetric groups. The method can be used during minimization of ROBDDs of incompletely specified Boolean functions. We apply it as a preprocessing step of symmetric sifting presented by Panda (1994) and Moller (1994) and of techniques for ROBDD minimization of incompletely specified Boolean functions presented by Chang (1994) and Shiple (1994). The technique is shown to be very effective: it improves ROBDD sizes of symmetric sifting by a factor of 51% and by a factor of 70% in combination with a slightly modified version of the technique of Chang and Shiple.