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2016 | OriginalPaper | Buchkapitel

20. Complete Problems

verfasst von : Bernhard Reus

Erschienen in: Limits of Computation

Verlag: Springer International Publishing

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Abstract

In this chapter we consider various examples of NP-complete problems. A famous result by Cook and Levin will provide us with a very first NP-complete problem. To this problem one can then apply reduction to show that other problems are NP-hard as well. Several such NP-complete problems are introduced: reachability of a (favourable) position for many interesting games, (unrestricted) database queries, and policy based routing. It turns out that most optimisation problems that are not in P are NP-complete. But there are also so-called NP-intermediate problems, for which we do not know whether they are NP-complete nor whether they are in P. Finally, will briefly discuss the concept of completeness for other complexity classes, i.e. P and semi-decidable problems.

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Fußnoten
1
There are so-called “truth tables” for each Boolean operator that fully explain its semantics.
 
2
This version of the problem is also sometimes called CNFSAT, when SAT is used to denote the problem for unrestricted propositional formulae.
 
3
The actual number of different variables is then less than or equal n.
 
4
Leonid Levin (born November 2, 1948) is a Soviet-American computer scientist who has apparently proved the same theorem as Stephen Cook independently behind the (then still existing) “iron curtain”.
 
5
A more detailed proof can be found in many textbooks.
 
6
There are many examples of this: consider Max-Cut versus Min-Cut or Integer Programming versus Linear Programming, TSP versus Postman Problem, and so on.
 
7
Recall that a consequence of this is that if any of those problems were in P we could deduce P = NP.
 
8
Karp [16] actually showed SAT \(\le _P\) Clique \(\le _P\) Node Cover \(\le _P\) Directed Hamiltonian Circuit \(\le _P\) Undirected Hamiltonian Circuit.
 
9
Karp [16] named this problem “Chromatic Number”.
 
10
Karp [16] actually showed that Graph Colouring \(\le _P\) Exact Cover \(\le _P\) 0-1Knapsack.
 
11
Karp [16] actually showed that 0-1 Knapsack \(\le _P\) Partition \(\le _P\) Max-Cut.
 
12
[14, Chap. 25.6].
 
13
Bejeweled TM is a trademark of PopCap Games and Candy Crush TM is a trademark of King.com Ltd.
 
14
Non-chess players are encouraged to look them up and play this classic game.
 
15
Without the 50 move rule that says that if no pawn was moved and no piece captured in the last 50 moves then it’s a draw.
 
16
Of course, not all these positions are legal positions but this can be ignored for this approximation.
 
17
In conclusion, [27] raises another interesting question: Since hundreds of millions of hours globally are spent playing Candy CrushTM and similar games, could one profit from this by using the manpower to solve other NP-complete problems hidden in the games?
 
18
Moshe Ya’akov Vardi is an Israeli computer scientist who teaches at Rice University in the US. His research spans many fields of computer science, including database systems, complexity theory, and program specification and verification. He has won numerous awards.
 
19
In a relational database system there is an optimizer which tries to avoid exactly this. And of course, real-life queries are—most of the time—not very large.
 
20
i.e. returning the same result as the original query.
 
21
So \(\mathcal{P}((u,v),p)\) can be interpreted as the set of rules of network device (router) u for destination v depending on the packet p to be forwarded. Usually, only (parts of) the header of packet p is considered, but not its payload.
 
22
We won’t go into the details of Classless Inter-Domain Routing (CIDR) address notation taught in every Network course.
 
23
This protocol still routes most traffic today despite the advent of IPv6.
 
24
Limbo denotes an uncertain period of awaiting a decision or resolution. The name origins in theology where it refers to a certain part of “hell” but is now used generally in the aforementioned sense.
 
25
László Babai (born July 20, 1950) is an award-winning Hungarian mathematician and computer scientist from the University of Chicago.
 
26
But the proof was not peer-reviewed yet at the time of writing.
 
27
\(2^{c \log _2 n} = (2^{\log _2 n})^c = n^c\).
 
28
The empty language is the empty set of words, \(\emptyset \), so it does not contain any words at all. Admittedly, not a very useful language, but that makes it even more important to be able to check efficiently that one’s grammar does not describe this language.
 
29
This had already been observed by [16] who uses some of the results in [3].
 
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Metadaten
Titel
Complete Problems
verfasst von
Bernhard Reus
Copyright-Jahr
2016
DOI
https://doi.org/10.1007/978-3-319-27889-6_20