  Descriptional complexity is the study of the conciseness of the various
models representing formal languages. The state complexity of a regular
language is the size, measured by the number of states of the smallest, either
deterministic or nondeterministic, finite automaton that recognises it.
Operational state complexity is the study of the state complexity of operations
over languages. In this survey, we review the state complexities of individual
regularity preserving language operations on regular and some subregular
languages. Then we revisit the state complexities of the combination of
individual operations. We also review methods of estimation and approximation
of state complexity of more complex combined operations.
