Euler product




In number theory, an Euler product is an expansion of a Dirichlet series into an infinite product indexed by prime numbers. The original such product was given for the sum of all positive integers raised to a certain power as proven by Leonhard Euler. This series and its continuation to the entire complex plane would later become known as the Riemann zeta function.




Contents






  • 1 Definition


  • 2 Convergence


  • 3 Examples


  • 4 Notable constants


  • 5 Notes


  • 6 References


  • 7 External links





Definition


In general, if a{displaystyle a}a is a multiplicative function, then the Dirichlet series


na(n)n−s{displaystyle sum _{n}a(n)n^{-s},}sum _{n}a(n)n^{-s},

is equal to


pP(p,s){displaystyle prod _{p}P(p,s),}prod _{p}P(p,s),

where the product is taken over prime numbers p{displaystyle p}p, and P(p,s){displaystyle P(p,s)}P(p,s) is the sum


1+a(p)p−s+a(p2)p−2s+⋯.{displaystyle 1+a(p)p^{-s}+a(p^{2})p^{-2s}+cdots .}1+a(p)p^{-s}+a(p^{2})p^{-2s}+cdots .

In fact, if we consider these as formal generating functions, the existence of such a formal Euler product expansion is a necessary and sufficient condition that a(n){displaystyle a(n)}a(n) be multiplicative: this says exactly that a(n){displaystyle a(n)}a(n) is the product of the a(pk){displaystyle a(p^{k})}a(p^{k}) whenever n{displaystyle n}n factors as the product of the powers pk{displaystyle p^{k}}p^{k} of distinct primes p{displaystyle p}p.


An important special case is that in which a(n){displaystyle a(n)}a(n) is totally multiplicative, so that P(p,s){displaystyle P(p,s)}P(p,s) is a geometric series. Then


P(p,s)=11−a(p)p−s,{displaystyle P(p,s)={frac {1}{1-a(p)p^{-s}}},}P(p,s)={frac {1}{1-a(p)p^{-s}}},

as is the case for the Riemann zeta-function, where a(n)=1{displaystyle a(n)=1}a(n)=1, and more generally for Dirichlet characters.



Convergence


In practice all the important cases are such that the infinite series and infinite product expansions are absolutely convergent in some region


Re⁡(s)>C,{displaystyle operatorname {Re} (s)>C,}{displaystyle operatorname {Re} (s)>C,}

that is, in some right half-plane in the complex numbers. This already gives some information, since the infinite product, to converge, must give a non-zero value; hence the function given by the infinite series is not zero in such a half-plane.


In the theory of modular forms it is typical to have Euler products with quadratic polynomials in the denominator here. The general Langlands philosophy includes a comparable explanation of the connection of polynomials of degree m, and the representation theory for GLm.



Examples


The Euler product attached to the Riemann zeta function ζ(s),{displaystyle zeta (s),}{displaystyle zeta (s),} using also the sum of the geometric series, is


p(1−p−s)−1=∏p(∑n=0∞p−ns)=∑n=1∞1ns=ζ(s).{displaystyle prod _{p}(1-p^{-s})^{-1}=prod _{p}{Big (}sum _{n=0}^{infty }p^{-ns}{Big )}=sum _{n=1}^{infty }{frac {1}{n^{s}}}=zeta (s).}{displaystyle prod _{p}(1-p^{-s})^{-1}=prod _{p}{Big (}sum _{n=0}^{infty }p^{-ns}{Big )}=sum _{n=1}^{infty }{frac {1}{n^{s}}}=zeta (s).}

while for the Liouville function λ(n)=(−1)Ω(n),{displaystyle lambda (n)=(-1)^{Omega (n)},}{displaystyle lambda (n)=(-1)^{Omega (n)},} it is


p(1+p−s)−1=∑n=1∞λ(n)ns=ζ(2s)ζ(s).{displaystyle prod _{p}(1+p^{-s})^{-1}=sum _{n=1}^{infty }{frac {lambda (n)}{n^{s}}}={frac {zeta (2s)}{zeta (s)}}.}{displaystyle prod _{p}(1+p^{-s})^{-1}=sum _{n=1}^{infty }{frac {lambda (n)}{n^{s}}}={frac {zeta (2s)}{zeta (s)}}.}

Using their reciprocals, two Euler products for the Möbius function μ(n){displaystyle mu (n)}mu (n) are


p(1−p−s)=∑n=1∞μ(n)ns=1ζ(s){displaystyle prod _{p}(1-p^{-s})=sum _{n=1}^{infty }{frac {mu (n)}{n^{s}}}={frac {1}{zeta (s)}}}prod _{p}(1-p^{-s})=sum _{n=1}^{infty }{frac {mu (n)}{n^{s}}}={frac {1}{zeta (s)}}

and


p(1+p−s)=∑n=1∞(n)|ns=ζ(s)ζ(2s).{displaystyle prod _{p}(1+p^{-s})=sum _{n=1}^{infty }{frac {|mu (n)|}{n^{s}}}={frac {zeta (s)}{zeta (2s)}}.}{displaystyle prod _{p}(1+p^{-s})=sum _{n=1}^{infty }{frac {|mu (n)|}{n^{s}}}={frac {zeta (s)}{zeta (2s)}}.}

Taking the ratio of these two gives


p(1+p−s1−p−s)=∏p(ps+1ps−1)=ζ(s)2ζ(2s).{displaystyle prod _{p}{Big (}{frac {1+p^{-s}}{1-p^{-s}}}{Big )}=prod _{p}{Big (}{frac {p^{s}+1}{p^{s}-1}}{Big )}={frac {zeta (s)^{2}}{zeta (2s)}}.}{displaystyle prod _{p}{Big (}{frac {1+p^{-s}}{1-p^{-s}}}{Big )}=prod _{p}{Big (}{frac {p^{s}+1}{p^{s}-1}}{Big )}={frac {zeta (s)^{2}}{zeta (2s)}}.}

Since for even s the Riemann zeta function ζ(s){displaystyle zeta (s)}zeta (s) has an analytic expression in terms of a rational multiple of πs,{displaystyle pi ^{s},}{displaystyle pi ^{s},} then for even exponents, this infinite product evaluates to a rational number. For example, since ζ(2)=π2/6,{displaystyle zeta (2)=pi ^{2}/6,}{displaystyle zeta (2)=pi ^{2}/6,} ζ(4)=π4/90,{displaystyle zeta (4)=pi ^{4}/90,}zeta (4)=pi ^{4}/90, and ζ(8)=π8/9450,{displaystyle zeta (8)=pi ^{8}/9450,}{displaystyle zeta (8)=pi ^{8}/9450,} then


p(p2+1p2−1)=52,{displaystyle prod _{p}{Big (}{frac {p^{2}+1}{p^{2}-1}}{Big )}={frac {5}{2}},}{displaystyle prod _{p}{Big (}{frac {p^{2}+1}{p^{2}-1}}{Big )}={frac {5}{2}},}

p(p4+1p4−1)=76,{displaystyle prod _{p}{Big (}{frac {p^{4}+1}{p^{4}-1}}{Big )}={frac {7}{6}},}{displaystyle prod _{p}{Big (}{frac {p^{4}+1}{p^{4}-1}}{Big )}={frac {7}{6}},}

and so on, with the first result known by Ramanujan. This family of infinite products is also equivalent to


p(1+2p−s+2p−2s+⋯)=∑n=1∞(n)n−s=ζ(s)2ζ(2s),{displaystyle prod _{p}(1+2p^{-s}+2p^{-2s}+cdots )=sum _{n=1}^{infty }2^{omega (n)}n^{-s}={frac {zeta (s)^{2}}{zeta (2s)}},}{displaystyle prod _{p}(1+2p^{-s}+2p^{-2s}+cdots )=sum _{n=1}^{infty }2^{omega (n)}n^{-s}={frac {zeta (s)^{2}}{zeta (2s)}},}

where ω(n){displaystyle omega (n)}omega (n) counts the number of distinct prime factors of n, and (n){displaystyle 2^{omega (n)}}2^{omega (n)} is the number of square-free divisors.


If χ(n){displaystyle chi (n)}chi (n) is a Dirichlet character of conductor N,{displaystyle N,}N, so that χ{displaystyle chi }chi is totally multiplicative and χ(n){displaystyle chi (n)}chi (n) only depends on n modulo N, and χ(n)=0{displaystyle chi (n)=0}chi (n)=0 if n is not coprime to N, then


p(1−χ(p)p−s)−1=∑n=1∞χ(n)n−s.{displaystyle prod _{p}(1-chi (p)p^{-s})^{-1}=sum _{n=1}^{infty }chi (n)n^{-s}.}{displaystyle prod _{p}(1-chi (p)p^{-s})^{-1}=sum _{n=1}^{infty }chi (n)n^{-s}.}

Here it is convenient to omit the primes p dividing the conductor N from the product. In his notebooks, Ramanujan generalized the Euler product for the zeta function as


p(x−p−s)≈1Lis⁡(x){displaystyle prod _{p}(x-p^{-s})approx {frac {1}{operatorname {Li} _{s}(x)}}}prod _{p}(x-p^{-s})approx {frac {1}{operatorname {Li} _{s}(x)}}

for s>1{displaystyle s>1}s>1 where Lis⁡(x){displaystyle operatorname {Li} _{s}(x)}operatorname {Li} _{s}(x) is the polylogarithm. For x=1{displaystyle x=1}x=1 the product above is just 1/ζ(s).{displaystyle 1/zeta (s).}1/zeta (s).



Notable constants


Many well known constants have Euler product expansions.


The Leibniz formula for π,


π4=∑n=0∞(−1)n2n+1=1−13+15−17+⋯,{displaystyle {frac {pi }{4}}=sum _{n=0}^{infty }{frac {(-1)^{n}}{2n+1}}=1-{frac {1}{3}}+{frac {1}{5}}-{frac {1}{7}}+cdots ,}{displaystyle {frac {pi }{4}}=sum _{n=0}^{infty }{frac {(-1)^{n}}{2n+1}}=1-{frac {1}{3}}+{frac {1}{5}}-{frac {1}{7}}+cdots ,}

can be interpreted as a Dirichlet series using the (unique) Dirichlet character modulo 4, and converted to an Euler product of superparticular ratios


π4=(∏p≡1(mod4)pp−1)⋅(∏p≡3(mod4)pp+1)=34⋅54⋅78⋅1112⋅1312⋯,{displaystyle {frac {pi }{4}}=left(prod _{pequiv 1{pmod {4}}}{frac {p}{p-1}}right)cdot left(prod _{pequiv 3{pmod {4}}}{frac {p}{p+1}}right)={frac {3}{4}}cdot {frac {5}{4}}cdot {frac {7}{8}}cdot {frac {11}{12}}cdot {frac {13}{12}}cdots ,}{displaystyle {frac {pi }{4}}=left(prod _{pequiv 1{pmod {4}}}{frac {p}{p-1}}right)cdot left(prod _{pequiv 3{pmod {4}}}{frac {p}{p+1}}right)={frac {3}{4}}cdot {frac {5}{4}}cdot {frac {7}{8}}cdot {frac {11}{12}}cdot {frac {13}{12}}cdots ,}

where each numerator is a prime number and each denominator is the nearest multiple of four.[1]


Other Euler products for known constants include:


Hardy–Littlewood's twin prime constant:


p>2(1−1(p−1)2)=0.660161...{displaystyle prod _{p>2}left(1-{frac {1}{(p-1)^{2}}}right)=0.660161...}{displaystyle prod _{p>2}left(1-{frac {1}{(p-1)^{2}}}right)=0.660161...}

Landau-Ramanujan constant:


π4∏p≡1(mod4)(1−1p2)1/2=0.764223...{displaystyle {frac {pi }{4}}prod _{pequiv 1{pmod {4}}}left(1-{frac {1}{p^{2}}}right)^{1/2}=0.764223...}{displaystyle {frac {pi }{4}}prod _{pequiv 1{pmod {4}}}left(1-{frac {1}{p^{2}}}right)^{1/2}=0.764223...}

12∏p≡3(mod4)(1−1p2)−1/2=0.764223...{displaystyle {frac {1}{sqrt {2}}}prod _{pequiv 3{pmod {4}}}left(1-{frac {1}{p^{2}}}right)^{-1/2}=0.764223...}{displaystyle {frac {1}{sqrt {2}}}prod _{pequiv 3{pmod {4}}}left(1-{frac {1}{p^{2}}}right)^{-1/2}=0.764223...}

Murata's constant (sequence A065485 in the OEIS):


p(1+1(p−1)2)=2.826419...{displaystyle prod _{p}left(1+{frac {1}{(p-1)^{2}}}right)=2.826419...}{displaystyle prod _{p}left(1+{frac {1}{(p-1)^{2}}}right)=2.826419...}

Strongly carefree constant ×ζ(2)2{displaystyle times zeta (2)^{2}}times zeta (2)^{2} OEIS: A065472:


p(1−1(p+1)2)=0.775883...{displaystyle prod _{p}left(1-{frac {1}{(p+1)^{2}}}right)=0.775883...}{displaystyle prod _{p}left(1-{frac {1}{(p+1)^{2}}}right)=0.775883...}

Artin's constant OEIS: A005596:


p(1−1p(p−1))=0.373955...{displaystyle prod _{p}left(1-{frac {1}{p(p-1)}}right)=0.373955...}{displaystyle prod _{p}left(1-{frac {1}{p(p-1)}}right)=0.373955...}

Landau's totient constant OEIS: A082695:


p(1+1p(p−1))=3152π(3)=1.943596...{displaystyle prod _{p}left(1+{frac {1}{p(p-1)}}right)={frac {315}{2pi ^{4}}}zeta (3)=1.943596...}{displaystyle prod _{p}left(1+{frac {1}{p(p-1)}}right)={frac {315}{2pi ^{4}}}zeta (3)=1.943596...}

Carefree constant ×ζ(2){displaystyle times zeta (2)}times zeta (2) OEIS: A065463:


p(1−1p(p+1))=0.704442...{displaystyle prod _{p}left(1-{frac {1}{p(p+1)}}right)=0.704442...}{displaystyle prod _{p}left(1-{frac {1}{p(p+1)}}right)=0.704442...}

(with reciprocal) OEIS: A065489:


p(1+1p2+p−1)=1.419562...{displaystyle prod _{p}left(1+{frac {1}{p^{2}+p-1}}right)=1.419562...}{displaystyle prod _{p}left(1+{frac {1}{p^{2}+p-1}}right)=1.419562...}

Feller-Tornier constant OEIS: A065493:


12+12∏p(1−2p2)=0.661317...{displaystyle {frac {1}{2}}+{frac {1}{2}}prod _{p}left(1-{frac {2}{p^{2}}}right)=0.661317...}{displaystyle {frac {1}{2}}+{frac {1}{2}}prod _{p}left(1-{frac {2}{p^{2}}}right)=0.661317...}

Quadratic class number constant OEIS: A065465:


p(1−1p2(p+1))=0.881513...{displaystyle prod _{p}left(1-{frac {1}{p^{2}(p+1)}}right)=0.881513...}{displaystyle prod _{p}left(1-{frac {1}{p^{2}(p+1)}}right)=0.881513...}

Totient summatory constant OEIS: A065483:


p(1+1p2(p−1))=1.339784...{displaystyle prod _{p}left(1+{frac {1}{p^{2}(p-1)}}right)=1.339784...}{displaystyle prod _{p}left(1+{frac {1}{p^{2}(p-1)}}right)=1.339784...}

Sarnak's constant OEIS: A065476:


p>2(1−p+2p3)=0.723648...{displaystyle prod _{p>2}left(1-{frac {p+2}{p^{3}}}right)=0.723648...}{displaystyle prod _{p>2}left(1-{frac {p+2}{p^{3}}}right)=0.723648...}

Carefree constant OEIS: A065464:


p(1−2p−1p3)=0.428249...{displaystyle prod _{p}left(1-{frac {2p-1}{p^{3}}}right)=0.428249...}{displaystyle prod _{p}left(1-{frac {2p-1}{p^{3}}}right)=0.428249...}

Strongly carefree constant OEIS: A065473:


p(1−3p−2p3)=0.286747...{displaystyle prod _{p}left(1-{frac {3p-2}{p^{3}}}right)=0.286747...}{displaystyle prod _{p}left(1-{frac {3p-2}{p^{3}}}right)=0.286747...}

Stephens' constant OEIS: A065478:


p(1−pp3−1)=0.575959...{displaystyle prod _{p}left(1-{frac {p}{p^{3}-1}}right)=0.575959...}{displaystyle prod _{p}left(1-{frac {p}{p^{3}-1}}right)=0.575959...}

Barban's constant OEIS: A175640:


p(1+3p2−1p(p+1)(p2−1))=2.596536...{displaystyle prod _{p}left(1+{frac {3p^{2}-1}{p(p+1)(p^{2}-1)}}right)=2.596536...}{displaystyle prod _{p}left(1+{frac {3p^{2}-1}{p(p+1)(p^{2}-1)}}right)=2.596536...}

Taniguchi's constant OEIS: A175639:


p(1−3p3+2p4+1p5−1p6)=0.678234...{displaystyle prod _{p}left(1-{frac {3}{p^{3}}}+{frac {2}{p^{4}}}+{frac {1}{p^{5}}}-{frac {1}{p^{6}}}right)=0.678234...}{displaystyle prod _{p}left(1-{frac {3}{p^{3}}}+{frac {2}{p^{4}}}+{frac {1}{p^{5}}}-{frac {1}{p^{6}}}right)=0.678234...}

Heath-Brown and Moroz constant OEIS: A118228:


p(1−1p)7(1+7p+1p2)=0.0013176...{displaystyle prod _{p}left(1-{frac {1}{p}}right)^{7}left(1+{frac {7p+1}{p^{2}}}right)=0.0013176...}{displaystyle prod _{p}left(1-{frac {1}{p}}right)^{7}left(1+{frac {7p+1}{p^{2}}}right)=0.0013176...}


Notes





  1. ^ Debnath, Lokenath (2010), The Legacy of Leonhard Euler: A Tricentennial Tribute, World Scientific, p. 214, ISBN 9781848165267.mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"""""""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}.




References




  • G. Polya, Induction and Analogy in Mathematics Volume 1 Princeton University Press (1954) L.C. Card 53-6388 (A very accessible English translation of Euler's memoir regarding this "Most Extraordinary Law of the Numbers" appears starting on page 91)


  • Apostol, Tom M. (1976), Introduction to analytic number theory, Undergraduate Texts in Mathematics, New York-Heidelberg: Springer-Verlag, ISBN 978-0-387-90163-3, MR 0434929, Zbl 0335.10001
    (Provides an introductory discussion of the Euler product in the context of classical number theory.)


  • G.H. Hardy and E.M. Wright, An introduction to the theory of numbers, 5th ed., Oxford (1979)
    ISBN 0-19-853171-0 (Chapter 17 gives further examples.)

  • George E. Andrews, Bruce C. Berndt, Ramanujan's Lost Notebook: Part I, Springer (2005),
    ISBN 0-387-25529-X

  • G. Niklasch, Some number theoretical constants: 1000-digit values"



External links




  • "Euler product". PlanetMath.


  • Hazewinkel, Michiel, ed. (2001) [1994], "Euler product", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4

  • Weisstein, Eric W. "Euler Product". MathWorld.


  • Niklasch, G. (23 Aug 2002). "Some number-theoretical constants". Archived from the original on 12 Jun 2006.




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