## [Computational Complexity] Favorite Theorems: Efficient Computation

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• February Edition How do we formalize the notion of efficient computation? Two important papers from the 60 s suggest polynomial time algorithms are efficient
Message 1 of 1 , Mar 8, 2005
February Edition

How do we formalize the notion of efficient computation? Two important papers from the 60's suggest polynomial time algorithms are efficient though both caution against equating the concepts.

Paths, Trees and Flowers, Jack Edmonds, 1965.

The Intrinsic Computational Difficulty of Functions, Alan Cobham, 1964.

Edmonds paper will always be best known for giving the first efficient algorithm for matching on general graphs. But I list it here because of a section of his paper labeled "Digression". Edmonds talks about the difference between exponential and algebraic (polynomial) order though he cautions against any rigid criteria for efficiency.

An explanation is due on the use of the words "efficient algorithm"…I am not prepared to set up the machinery necessary to give it formal meaning, nor is the present context appropriate for doing this…For practical purposes the difference between algebraic and exponential order is more crucial than the difference between [computable and not computable]…It would be unfortunate for any rigid criterion to inhibit the practical development of algorithms which are either not known or known not to conform nicely to the criterion…However, if only to motivate the search for good, practical algorithms, it is important to realize that it is mathematically sensible even to question their existence.
Cobham defined the class we now call P as important because of its machine independence.
For several reasons the class P seems a natural one to consider. For one thing, if we formalize the definition relative to various general classes of computing machines we seem always to end up with the same well-defined class of functions. Thus we can give a mathematical characterization of P having some confidence it characterizes correctly our informally defined class. This class then turns out to have several natural closure properties, being closed in particular under explicit transformation, composition and limited recursion on notation (digit-by-digit recursion).
Cobham also offers a caution.
The problem is reminiscent of, and obviously closely related to, that of the formalization of the notion of effectiveness. But the emphasis is different in that the physical aspects of the computation process are here of predominant concern.
Perhaps Cobham realized there might be future models of computation that may not correspond to his class P. Later developments of randomized and quantum computation will show that perhaps we cannot have a fixed notion of efficient computation.

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Posted by Lance to Computational Complexity at 3/8/2005 02:32:00 PM

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