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[[#References|[a9]]] relates <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007016.png" /> to <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007017.png" />, the sum of the absolute values of the coefficients of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007018.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007019.png" /> denotes the total degree of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007020.png" />, i.e. the sum of the degrees in each variable separately. A recent inequality for polynomials of one variable is that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007021.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007022.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007023.png" /> is the sum of the degrees of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007024.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007025.png" />, and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007026.png" /> is the best possible constant [[#References|[a2]]].
 
[[#References|[a9]]] relates <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007016.png" /> to <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007017.png" />, the sum of the absolute values of the coefficients of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007018.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007019.png" /> denotes the total degree of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007020.png" />, i.e. the sum of the degrees in each variable separately. A recent inequality for polynomials of one variable is that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007021.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007022.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007023.png" /> is the sum of the degrees of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007024.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007025.png" />, and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007026.png" /> is the best possible constant [[#References|[a2]]].
  
Specializing to polynomials with integer coefficients, in case <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007027.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007028.png" /> is the logarithm of an algebraic integer (cf. [[Algebraic number|Algebraic number]]). If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007029.png" />, there are few explicit formulas known, but those that do exist suggest that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007030.png" /> has intimate connections with [[K-theory|<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007031.png" />-theory]]. For example, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007032.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007033.png" /> is the [[Dirichlet L-function|Dirichlet <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007034.png" />-function]] for the odd primitive character of conductor <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007035.png" />, i.e. <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007036.png" />, and it has been conjectured that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007037.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007038.png" /> is the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007039.png" />-function of an [[Elliptic curve|elliptic curve]] of conductor <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007040.png" />. This formula has not been proved but has been verified to over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007041.png" /> decimal places [[#References|[a3]]], [[#References|[a4]]].
+
Specializing to polynomials with integer coefficients, in case <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007027.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007028.png" /> is the logarithm of an [[algebraic integer]]. If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007029.png" />, there are few explicit formulas known, but those that do exist suggest that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007030.png" /> has intimate connections with [[K-theory|<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007031.png" />-theory]]. For example, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007032.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007033.png" /> is the [[Dirichlet L-function|Dirichlet <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007034.png" />-function]] for the odd primitive character of conductor <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007035.png" />, i.e. <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007036.png" />, and it has been conjectured that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007037.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007038.png" /> is the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007039.png" />-function of an [[Elliptic curve|elliptic curve]] of conductor <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007040.png" />. This formula has not been proved but has been verified to over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007041.png" /> decimal places [[#References|[a3]]], [[#References|[a4]]].
  
 
The Mahler measure <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007042.png" /> occurs naturally as the growth rate in many problems, for example as the entropy of certain <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007044.png" />-actions [[#References|[a10]]]. The set of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007045.png" /> for which <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007046.png" /> is known: in case <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007047.png" />, a theorem of Kronecker shows that these are products of cyclotomic polynomials and monomials. In case <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007048.png" />, these are the generalized cyclotomic polynomials [[#References|[a1]]]. An important open question, known as Lehmer's problem, is whether there is a constant <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007049.png" /> such that if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007050.png" />, then <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007051.png" />. This is known to be the case if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007052.png" /> is a non-reciprocal polynomial, where a polynomial is reciprocal if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007053.png" /> is a monomial. In this case, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007054.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007055.png" /> is the smallest [[Pisot number|Pisot number]], the real root of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007056.png" /> [[#References|[a6]]], [[#References|[a1]]]. A possible value for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007057.png" /> is <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007058.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007059.png" /> is the smallest known [[Salem number|Salem number]], a number of degree <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007060.png" /> known as Lehmer's number.
 
The Mahler measure <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007042.png" /> occurs naturally as the growth rate in many problems, for example as the entropy of certain <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007044.png" />-actions [[#References|[a10]]]. The set of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007045.png" /> for which <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007046.png" /> is known: in case <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007047.png" />, a theorem of Kronecker shows that these are products of cyclotomic polynomials and monomials. In case <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007048.png" />, these are the generalized cyclotomic polynomials [[#References|[a1]]]. An important open question, known as Lehmer's problem, is whether there is a constant <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007049.png" /> such that if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007050.png" />, then <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007051.png" />. This is known to be the case if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007052.png" /> is a non-reciprocal polynomial, where a polynomial is reciprocal if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007053.png" /> is a monomial. In this case, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007054.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007055.png" /> is the smallest [[Pisot number|Pisot number]], the real root of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007056.png" /> [[#References|[a6]]], [[#References|[a1]]]. A possible value for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007057.png" /> is <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007058.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007059.png" /> is the smallest known [[Salem number|Salem number]], a number of degree <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/m/m120/m120070/m12007060.png" /> known as Lehmer's number.

Revision as of 08:51, 11 December 2016

Given a polynomial with complex coefficients, the logarithmic Mahler measure is defined to be the average over the unit -torus of , i.e.

The Mahler measure is defined by , so that is the geometric mean of over the -torus. If and , Jensen's formula gives the explicit formula

so that .

The Mahler measure is useful in the study of polynomial inequalities because of the multiplicative property . The important basic inequality

[a9] relates to , the sum of the absolute values of the coefficients of , where denotes the total degree of , i.e. the sum of the degrees in each variable separately. A recent inequality for polynomials of one variable is that , where , is the sum of the degrees of and , and is the best possible constant [a2].

Specializing to polynomials with integer coefficients, in case , is the logarithm of an algebraic integer. If , there are few explicit formulas known, but those that do exist suggest that has intimate connections with -theory. For example, , where is the Dirichlet -function for the odd primitive character of conductor , i.e. , and it has been conjectured that , where is the -function of an elliptic curve of conductor . This formula has not been proved but has been verified to over decimal places [a3], [a4].

The Mahler measure occurs naturally as the growth rate in many problems, for example as the entropy of certain -actions [a10]. The set of for which is known: in case , a theorem of Kronecker shows that these are products of cyclotomic polynomials and monomials. In case , these are the generalized cyclotomic polynomials [a1]. An important open question, known as Lehmer's problem, is whether there is a constant such that if , then . This is known to be the case if is a non-reciprocal polynomial, where a polynomial is reciprocal if is a monomial. In this case, , where is the smallest Pisot number, the real root of [a6], [a1]. A possible value for is , where is the smallest known Salem number, a number of degree known as Lehmer's number.

For , the best result in this direction is that , where is an explicit absolute constant and is the degree of [a5]. A result that applies to polynomials in any number of variables is an explicit constant depending on the number of non-zero coefficients of such that [a7], [a1].

A recent development is the elliptic Mahler measure [a8], in which the torus is replaced by an elliptic curve. It seems likely that this will have an interpretation as the entropy of a dynamical system but this remains as of yet (1998) a future development.

References

[a1] D.W. Boyd, "Kronecker's theorem and Lehmer's problem for polynomials in several variables" J. Number Th. , 13 (1981) pp. 116–121
[a2] D.W. Boyd, "Two sharp inequalities for the norm of a factor of a polynomial" Mathematika , 39 (1992) pp. 341–349 MR1203290 Zbl 0758.30003
[a3] D.W. Boyd, "Mahler's measure and special values of -functions" Experim. Math. , 37 (1998) pp. 37–82
[a4] C. Deninger, "Deligne periods of mixed motives, -theory and the entropy of certain -actions" J. Amer. Math. Soc. , 10 (1997) pp. 259–281 MR1415320
[a5] E. Dobrowolski, "On a question of Lehmer and the number of irreducible factors of a polynomial" Acta Arith. , 34 (1979) pp. 391–401 MR0543210 Zbl 0416.12001
[a6] C.J. Smyth, "On the product of the conjugates outside the unit circle of an algebraic integer" Bull. London Math. Soc. , 3 (1971) pp. 169–175 MR0289451 Zbl 0235.12003
[a7] E. Dobrowolski, "Mahler's measure of a polynomial in function of the number of its coefficients" Canad. Math. Bull. , 34 (1991) pp. 186–195 MR1113295
[a8] G. Everest, Ni Fhlathúin Brid, "The elliptic Mahler measure" Math. Proc. Cambridge Philos. Soc. , 120 : 1 (1996) pp. 13–25 MR1373343 Zbl 0865.11068
[a9] K. Mahler, "On some inequalities for polynomials in several variables" J. London Math. Soc. , 37 : 2 (1962) pp. 341–344 MR0138593 Zbl 0105.06301
[a10] K. Schmidt, "Dynamical systems of algebraic origin" , Birkhäuser (1995) MR1345152 Zbl 0833.28001
How to Cite This Entry:
Mahler measure. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Mahler_measure&oldid=36175
This article was adapted from an original article by David Boyd (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article