A reactive system maintains an on-goint interaction with its environment. At each moment in time, the system receives from the environment an assignment to the input signals and responds with an assignment to the output signals. The system is correct with respect to a given specification if, for all environments, the infinite computation that is generated by the interaction between the system and the environment satisfies the specification. Reactive systems are modeled by transducers: finite state machines whose transitions are labeled by assignments to the input signals and whose states are labeled by assignments to the output signals. In the synthesis problem, we automatically transform a given specification into a correct transducer.

While complexity measures receive much attention in the design of algorithms, they are less explored in the context of synthesis. This is a serious drawback: just as we cannot imagine an algorithm designer that accepts a correct algorithm without checking its complexity, we cannot expect designers of reactive systems to accept synthesized systems that are only guaranteed to be correct. It is not clear, however, how to define the complexity of a transducer. Unlike the case of algorithms (or Turing machines in general), there is no "size of input" to relate to, and measures like time and space complexity are irrelevant. Indeed, we care for on-going interactions, along which the transducer reacts instantly according to its transition function. One immediate complexity measure for a transducer is its size, but many more complexity measures are of interest. The talk discusses such measures and describes how the search for optimal reactive systems affects the synthesis problem.