Editing P (complexity) (section)

==Properties==
Polynomial-time algorithms are closed under composition. Intuitively, this says that if one writes a function that is polynomial-time assuming that function calls are constant-time, and if those called functions themselves require polynomial time, then the entire algorithm takes polynomial time. One consequence of this is that P is [[low (complexity)|low]] for itself. This is also one of the main reasons that P is considered to be a machine-independent class; any machine "feature", such as [[random access]], that can be simulated in polynomial time can simply be composed with the main polynomial-time algorithm to reduce it to a polynomial-time algorithm on a more basic machine.

Languages in P are also closed under reversal, [[Intersection (set theory)|intersection]], [[Union (set theory)|union]], [[concatenation]], [[Kleene closure]], inverse [[homomorphism]], and [[Complement (complexity)|complementation]].<ref>{{cite book|last=Hopcroft|first=John E.|title=Introduction to automata theory, languages, and computation|year=2001|publisher=Addison-Wesley|location=Boston|isbn=978-0201441246|pages=425–426|edition=2.|author2=Rajeev Motwani |author3=Jeffrey D. Ullman }}</ref>