formulasearchengine - User contributions [en]

Tree (graph theory)

2014-01-02T16:40:43Z

108.68.138.161: /* Unlabeled trees */ Flajolet & Sedgewick give an algorithm giving the constants to arbitrary precision

In [[number theory]], a '''Gaussian integer''' is a [[complex number]] whose real and imaginary part are both [[integer]]s. The Gaussian integers, with ordinary [[addition]] and [[multiplication]] of [[complex numbers]], form an [[integral domain]], usually written as '''Z'''[''i'']. This integral domain is a particular case of a [[commutative ring]] of [[quadratic integer]]s. It does not have a [[total order]]ing that respects arithmetic.

[[Image:Gaussian integer lattice.png|thumb|217px|Gaussian integers as [[lattice point]]s in the [[complex plane]]]]

Formally, Gaussian integers are the set

:<math>\mathbb{Z}[i]=\{a+bi \mid a,b\in \mathbb{Z} \},\ \mbox{where}\ i^2 = -1.</math>

Note that when they are considered within the [[complex plane]] the Gaussian integers may be seen to constitute the 2-dimensional [[integer lattice]].

The [[Field norm|''(arithmetic or field) norm'']] of a Gaussian integer is the square of its absolute value (Euclidean norm) as a complex number and a [[natural number]] defined as

:<math>N \left(a+bi \right) = a^2+b^2 = (a+bi)\overline{(a+bi)} = (a+bi)(a-bi).</math>

(Where the overline over "a+bi" refers to the [[complex conjugate]].)

The norm is [[completely multiplicative function|multiplicative]], since the absolute value of complex numbers is multiplicative, i.e., one has

:<math>N(z\cdot w) = N(z)\cdot N(w).</math>

The latter can also be verified by a straightforward check. The [[unit (ring theory)|unit]]s of '''Z'''[''i''] are precisely those elements with norm 1, i.e. the elements 1, −1, ''i'' and −''i''.

==As a principal ideal domain==
The Gaussian integers form a [[principal ideal domain]] with [[unit (ring theory)|unit]]s 1, −1, ''i'', and −''i''. If ''x'' is a Gaussian integer, the four numbers ''x'', ''ix'', −''x'', and −''ix'' are called the associates of ''x''. As for every [[principal ideal domain]], the Gaussian integers form also a [[unique factorization domain]].

The [[prime element]]s of '''Z'''[''i''] are also known as '''Gaussian primes'''. An associate of a Gaussian prime is also a Gaussian prime. The Gaussian primes are symmetric about the real and imaginary axes. The ''positive integer'' Gaussian primes are the [[prime numbers]] [[Congruence class|congruent to]] 3 modulo 4, {{OEIS|A002145}}. One should not refer to only these numbers as "the Gaussian primes", which refers to ''all'' the Gaussian primes, many of which do not lie in '''Z'''.<ref name="Gaussian Primes naming error">[http://oeis.org/A002145#COMMENT], OEIS sequence A002145 "COMMENT" section</ref>

A Gaussian integer <math>a+bi</math> is a Gaussian prime if and only if either:
* one of ''a'', ''b'' is zero and the other is a prime number of the form <math>4n+3</math> (with ''n'' a nonnegative integer) or its negative <math>-(4n+3)</math>, or
* both are nonzero and <math>a^2+b^2</math> is a prime number (which will ''not'' be of the form <math>4n+3</math>).

The following elaborates on these conditions.

2 is a special case (in the language of [[algebraic number theory]], 2 is the only [[Ramification#In_algebraic_number_theory|ramified]] prime in '''Z'''[''i'']).

The integer 2 factors as <math>2=(1+i)(1-i)=i(1-i)^2</math> as a Gaussian integer, the second factorisation (in which ''i'' is a unit) showing that 2 is divisible by the square of a Gaussian prime; it is the unique prime number with this property.

The necessary conditions can be stated as following: if a Gaussian integer is a Gaussian prime, then either its norm is a prime number, or its norm is a square of a prime number. This is because for any Gaussian integer <math>g</math>, notice
:<math>g \mid g\bar{g} =N(g)</math>.

Here <math>\mid</math> means “divides”; that is, <math>x \mid y</math> if <math>x</math> is a divisor of <math>y</math>.

Now <math>N(g)</math> is an integer, and so can be factored as a product <math>p_{1}p_{2}\cdots p_{n}</math> of prime numbers, by the [[fundamental theorem of arithmetic]]. By definition of prime element, if <math>g</math> is a Gaussian prime, then it divides (in '''Z'''[''i'']) some <math>p_i</math>. Also, <math>\bar g</math> divides
:<math>\overline{p_i}=p_i</math>, so <math>N(g) = g\bar{g} \mid p_{i}^{2}</math> in '''Z'''.

This gives only two options: either the norm of <math>g</math> is a prime number, or the square of a prime number.

If in fact <math>N(g)=p^2</math> for some prime number <math>p</math>, then both <math>g</math> and <math>\overline{g}</math> divide <math>p^2</math>. Neither can be a unit, and so
:<math>g=pu</math> and <math>\overline{g}=p\overline{u}</math>

where <math>u</math> is a unit. This is to say that either <math>a=0</math> or <math>b=0</math>, where <math>g=a+bi</math>.

However, not every prime number <math>p</math> is a Gaussian prime. 2 is not because <math>2=(1+i)(1-i)</math>. Neither are prime numbers of the form <math>4n+1</math> because [[Fermat's theorem on sums of two squares]] assures us they can be written <math>a^2+b^2</math> for integers <math>a</math> and <math>b</math>, and <math>a^2+b^2 = (a+bi)(a-bi)</math>. The only type of prime numbers remaining are of the form <math>4n+3</math>.

Prime numbers of the form <math>4n+3</math> are also Gaussian primes. For suppose <math>g=p+0i</math> for <math>p=4n+3</math>, and it can be factored <math>g=hk</math>. Then <math>p^2=N(g)=N(h)N(k)</math>. If the factorization is non-trivial, then <math>N(h)=N(k)=p</math>. But no sum of squares of integers can be written <math>4n+3</math>. So the factorization must have been trivial and <math>g</math> is a Gaussian prime.

If <math>g</math> is a Gaussian integer whose norm is a prime number, then <math>g</math> is a Gaussian prime, because the norm is multiplicative.

===As an integral closure===
The ring of Gaussian integers is the [[integral closure]] of '''Z''' in the [[field (mathematics)|field]] of [[Gaussian rational]]s '''Q'''(''i'') consisting of the complex numbers whose real and imaginary part are both [[rational number|rational]].

===As a Euclidean domain===
It is easy to see graphically that every [[complex number]] is within <math>\frac{\sqrt 2}{2}</math> units of a Gaussian integer.

Put another way, every complex number (and hence every Gaussian integer) has a maximal distance of
:<math>\frac{\sqrt 2}{2}\sqrt{N(z)}</math>

units to some multiple of z, where z is any Gaussian integer; this turns '''Z'''[''i''] into a [[Euclidean domain]], where
:<math>v(z) = N(z). \,</math>

==Historical background==

The ring of Gaussian integers was introduced by [[Carl Friedrich Gauss]] in his second monograph on [[quartic reciprocity]] (1832) (see [http://www.ems-ph.org/journals/show_pdf.php?issn=0013-6018&vol=53&iss=1&rank=2]). The theorem of [[quadratic reciprocity]] (which he had first succeeded in proving in 1796) relates the solvability of the congruence ''x''2 ≡ ''q'' (mod ''p'') to that of ''x''2 ≡ ''p'' (mod ''q''). Similarly, cubic reciprocity relates the solvability of ''x''3 ≡ ''q'' (mod ''p'') to that of ''x''3 ≡ ''p'' (mod ''q''), and biquadratic (or quartic) reciprocity is a relation between ''x''4 ≡ ''q'' (mod ''p'') and ''x''4 ≡ ''p'' (mod ''q''). Gauss discovered that the law of biquadratic reciprocity and its supplements were more easily stated and proved as statements about "whole complex numbers" (i.e. the Gaussian integers) than they are as statements about ordinary whole numbers (i.e. the integers).

In a footnote he notes that the [[Eisenstein integer]]s are the natural domain for stating and proving results on [[cubic reciprocity]] and indicates that similar extensions of the integers are the appropriate domains for studying higher reciprocity laws.

This paper not only introduced the Gaussian integers and proved they are a unique factorization domain, it also introduced the terms norm, unit, primary, and associate, which are now standard in algebraic number theory.

==Unsolved problems==

[[Image:gauss-primes-768x768.png|170px|thumb|Repartition in the plane of the small Gaussian primes]]

Most of the unsolved problems are related to the repartition in the plane of the Gaussian primes.

* [[Gauss's circle problem]] does not deal with the Gaussian integers ''per se'', but instead asks for the number of [[lattice point]]s inside a circle of a given radius centered at the origin. This is equivalent to determining the number of Gaussian integers with norm less than a given value.

There are also conjectures and unsolved problems about the Gaussian primes. Two of them are:

* The real and imaginary axes have the infinite set of Gaussian primes 3, 7, 11, 19, ... and their associates. Are there any other lines that have infinitely many Gaussian primes on them? In particular, are there infinitely many Gaussian primes of the form 1+''ki''?<ref>Ribenboim, Ch.III.4.D Ch. 6.II, Ch. 6.IV (Hardy & Littlewood's conjecture E and F)</ref>

* Is it possible to walk to infinity using the Gaussian primes as stepping stones and taking steps of bounded length? This is known as the [[Gaussian moat]] problem; it was posed in 1962 by [[Basil Gordon]] and remains unsolved.<ref>{{cite journal
| last1 = Gethner | first1 = Ellen
| last2 = Wagon | first2 = Stan | author2-link = Stan Wagon
| last3 = Wick | first3 = Brian
| doi = 10.2307/2589708
| issue = 4
| journal = [[The American Mathematical Monthly]]
| mr = 1614871 | zbl=0946.11002
| pages = 327–337
| title = A stroll through the Gaussian primes
| volume = 105
| year = 1998}}</ref><ref>{{cite book |last=Guy | first=Richard K. | authorlink=Richard K. Guy | title=Unsolved problems in number theory | publisher=[[Springer-Verlag]] |edition=3rd | year=2004 |isbn=978-0-387-20860-2 | zbl=1058.11001 | pages=55–57}}</ref>

==See also==
* [[Quadratic integer]]
* [[Hurwitz quaternion]]
* [[Eisenstein integer]]
* [[Algebraic integer]]
* [[Kummer ring]]
* [[Proofs of Fermat's theorem on sums of two squares]]
* [[Proofs of quadratic reciprocity]]
* [[Splitting of prime ideals in Galois extensions]] describes the structure of prime ideals in the Gaussian integers
* [[Table of Gaussian integer factorizations]]

==Notes==

{{reflist}}

==References==
* C. F. Gauss, Theoria residuorum biquadraticorum. Commentatio secunda., Comm. Soc. Reg. Sci. Göttingen 7 (1832) 1-34; reprinted in Werke, Georg Olms Verlag, Hildesheim, 1973, pp. 93–148.

* {{cite journal | url=http://www.ems-ph.org/journals/show_pdf.php?issn=0013-6018&vol=53&iss=1&rank=2 | title=From Numbers to Rings: The Early History of Ring Theory | first1=Israel | last1=Kleiner | journal=Elem. Math. | volume=53 |number=1 | doi=10.1007/s000170050029 | year=1998 | pages=18–35 | zbl=0908.16001 }}
*{{cite book | last1 = Ribenboim | first1 = Paulo | authorlink=Paulo Ribenboim | title = The New Book of Prime Number Records | edition=3rd | publisher= Springer | location = New York | date = 1996 | isbn = 0-387-94457-5 | zbl=0856.11001 }}

==External links==
* [http://www.alpertron.com.ar/GAUSSIAN.HTM www.alpertron.com.ar/GAUSSIAN.HTM] is a Java applet that evaluates expressions containing Gaussian integers and factors them into Gaussian primes.
* [http://www.alpertron.com.ar/GAUSSPR.HTM www.alpertron.com.ar/GAUSSPR.HTM] is a Java applet that features a graphical view of Gaussian primes.
* Henry G. Baker (1993) Complex Gaussian Integers for 'Gaussian Graphics', ACM SIGPLAN Notices, Vol. 28, Issue 11. [http://portal.acm.org/citation.cfm?doid=165564.165571 DOI 10.1145/165564.165571] [http://home.pipeline.com/~hbaker1/Gaussian.html (html)]
* [http://www.imocompendium.com/index.php?options=mbb|tekstkut&page=0&art=extensions_ddj|f&ttn=Dushan%20D;jukic1|%20Arithmetic%20in%20Quadratic%20Fields|N/A&knj=&p=3nbbw45001 IMO Compendium] text on quadratic extensions and Gaussian Integers in problem solving
* {{Mathworld|title= Landau's Problems|urlname= LandausProblems}}

{{Prime number classes}}

[[Category:Cyclotomic fields]]
[[Category:Algebraic numbers]]
[[Category:Lattice points]]

{{Link GA|ru}}

Differential geometry of surfaces

2012-08-17T16:21:58Z

108.68.101.33: Disambiguated: complex structure → Linear complex structure; Help needed: Index (mathematics), Torsion-free, Lattice (mathematics)

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Continuous quantum computation

2011-02-25T04:03:22Z

108.68.109.77:

[[Image:Implication graph.svg|thumb|360px|An implication graph representing the [[2-satisfiability]] instance <math>\scriptscriptstyle(x_0\lor x_2)\land(x_0\lor\lnot x_3)\land(x_1\lor\lnot x_3)\land(x_1\lor\lnot x_4)\land(x_2\lor\lnot x_4)\land{}\atop\quad\scriptscriptstyle(x_0\lor\lnot x_5)\land (x_1\lor\lnot x_5)\land (x_2\lor\lnot x_5)\land (x_3\lor x_6)\land (x_4\lor x_6)\land (x_5\lor x_6).</math>]]
In [[mathematical logic]], an '''implication graph''' is a [[skew-symmetric graph|skew-symmetric]] [[directed graph]] ''G''(''V'', ''E'') composed of vertex set ''V'' and directed edge set ''E''. Each vertex in ''V'' represents the truth status of a [[Boolean literal]], and each directed edge from vertex ''u'' to vertex ''v'' represents the [[material implication]] "If the literal ''u'' is true then the literal ''v'' is also true". Implication graphs were originally used for analyzing complex [[Boolean expression]]s.

==Applications==
A [[2-satisfiability]] instance in [[conjunctive normal form]] can be transformed into an implication graph by replacing each of its [[disjunction]]s by a pair of implications. An instance is satisfiable if and only if no literal and its negation belong to the same [[strongly connected component]] of its implication graph; this characterization can be used to solve 2-satisfiability instances in linear time.<ref>{{cite journal|author = Aspvall, Bengt; [[Michael Plass|Plass, Michael F.]]; [[Robert Tarjan|Tarjan, Robert E.]]|title = A linear-time algorithm for testing the truth of certain quantified boolean formulas|journal = Information Processing Letters | volume = 8 | issue = 3 | pages = 121–123|year = 1979|doi = 10.1016/0020-0190(79)90002-4}}</ref>

==References==
<references/>

[[Category:Boolean algebra]]
[[Category:Application-specific graphs]]
[[Category:Directed graphs]]
[[Category:Graph families]]