Coase theorem
From Academic Kids

In law and economics, the Coase theorem, attributed to Ronald Coase, is a theorem relating to the economic efficiency of a government's allocation of property rights. In essence, the theorem states that in the absence of transaction costs, all government allocations of property are equally efficient, because interested parties will bargain privately to correct any externality. As a corollary, the theorem also implies that in the presence of transaction costs, government may minimize inefficiency by allocating property initially to the party assigning it the greatest utility. This theorem, which earned Coase the 1991 Nobel Prize in Economics, is an important basis for most modern economic analyses of government regulation.
What Coase originally proposed in 1959 in the context of the regulation of radio frequencies was that as long as property rights in these frequencies were well defined, it ultimately did not matter if adjacent radio stations would initially interfere with each other by broadcasting in the same frequency band. The station able to reap the higher economic gain of the two from broadcasting would in this case have an incentive to pay the other station not to interfere. In the absence of transaction costs, both stations would strike a mutually advantageous deal. Put differently, it would not matter whether one or the other station had the initial right to broadcast; eventually, the right to broadcast would end up with the party that was able to put it to the most profitable use.
Coase's main point, clarified in an article published in 1960 (Coase 1960) and cited when he was awarded the Nobel Prize in 1991, was that transaction costs, however, could not be neglected, and that therefore, the initial allocation of property rights mattered in the presence of side effects (externalities).
George Stigler summarised the resolution of the externality problem in the absence of transaction costs in a 1966 economics textbook in terms of private and social cost, and for the first time called it a "theorem". Nevertheless, since the 1960s, a voluminous literature on the Coase theorem and its various interpretations, proofs, and refutations, has developed that continues to grow.
References
 Coase, Ronald H. The Problem of Social Cost. J. Law & Econ. 3, p. 1 (1960) (http://www.sfu.ca/~allen/CoaseJLE1960.pdf).
 A statement and proof of a simple mathematical version of the theorem (http://www.sims.berkeley.edu/academics/courses/is231/f04/is231Coase.htm).
External links
 Ronald Coase and the Coase Theorem (http://www.huppi.com/kangaroo/Lchicoase.htm) A critical look.
de:CoaseTheorem es:Teorema de Coase it:Teorema di Coase tr:Coase Teoremi