Some results by Lawton and Boyd

From the definition, the Mahler measure is viewed as the integrated values of polynomials over the torus (also see Lehmer's conjecture). If ${\displaystyle p}$ vanishes on the torus ${\displaystyle (S^{1})^{n}}$, then the convergence of the integral defining ${\displaystyle M(p)}$ is not obvious, but it is known that ${\displaystyle M(p)}$ does converge and is equal to a limit of one-variable Mahler measures,^[4] which had been conjectured by Boyd.^[5][6]

This is formulated as follows: Let ${\displaystyle \mathbb {Z} }$ denote the integers and define ${\displaystyle \mathbb {Z} _{+}^{N}=\{r=(r_{1},\dots ,r_{N})\in \mathbb {Z} ^{N}:r_{j}\geq 0\ {\text{for}}\ 1\leq j\leq N\}}$ . If ${\displaystyle Q(z_{1},\dots ,z_{N})}$ is a polynomial in ${\displaystyle N}$ variables and ${\displaystyle r=(r_{1},\dots ,r_{N})\in \mathbb {Z} _{+}^{N}}$ define the polynomial ${\displaystyle Q_{r}(z)}$ of one variable by

$${\displaystyle Q_{r}(z):=Q(z^{r_{1}},\dots ,z^{r_{N}})}$$

and define ${\displaystyle q(r)}$ by

$${\displaystyle q(r):=\min \left\{H(s):s=(s_{1},\dots ,s_{N})\in \mathbb {Z} ^{N},s\neq (0,\dots ,0)~{\text{and}}~\sum _{j=1}^{N}s_{j}r_{j}=0\right\}}$$

where ${\displaystyle H(s)=\max\{|s_{j}|:1\leq j\leq N\}}$.

Theorem (Lawton) — Let ${\displaystyle Q(z_{1},\dots ,z_{N})}$ be a polynomial in N variables with complex coefficients. Then the following limit is valid (even if the condition that ${\displaystyle r_{i}\geq 0}$ is relaxed):

$${\displaystyle \lim _{q(r)\to \infty }M(Q_{r})=M(Q)}$$

Boyd's proposal

Boyd provided more general statements than the above theorem. He pointed out that the classical Kronecker's theorem, which characterizes monic polynomials with integer coefficients all of whose roots are inside the unit disk, can be regarded as characterizing those polynomials of one variable whose measure is exactly 1, and that this result extends to polynomials in several variables.^[6]

Define an extended cyclotomic polynomial to be a polynomial of the form

$${\displaystyle \Psi (z)=z_{1}^{b_{1}}\dots z_{n}^{b_{n}}\Phi _{m}(z_{1}^{v_{1}}\dots z_{n}^{v_{n}}),}$$

where ${\displaystyle \Phi _{m}(z)}$ is the m-th cyclotomic polynomial, the ${\displaystyle v_{i}}$ are integers, and the ${\displaystyle b_{i}=\max(0,-v_{i}\deg \Phi _{m})}$ are chosen minimally so that ${\displaystyle \Psi (z)}$ is a polynomial in the ${\displaystyle z_{i}}$. Let ${\displaystyle K_{n}}$ be the set of polynomials that are products of monomials ${\displaystyle \pm z_{1}^{c_{1}}\dots z_{n}^{c_{n}}}$ and extended cyclotomic polynomials.

This led Boyd to consider the set of values

$${\displaystyle L_{n}:={\bigl \{}m(P(z_{1},\dots ,z_{n})):P\in \mathbb {Z} [z_{1},\dots ,z_{n}]{\bigr \}},}$$

and the union ${\textstyle {L}_{\infty }=\bigcup _{n=1}^{\infty }L_{n}}$. He made the far-reaching conjecture^[5] that the set of ${\displaystyle {L}_{\infty }}$ is a closed subset of ${\displaystyle \mathbb {R} }$. An immediate consequence of this conjecture would be the truth of Lehmer's conjecture, albeit without an explicit lower bound. As Smyth's result suggests that ${\displaystyle L_{1}\subsetneqq L_{2}}$ , Boyd further conjectures that

$${\displaystyle L_{1}\subsetneqq L_{2}\subsetneqq L_{3}\subsetneqq \ \cdots .}$$

Mahler measure and entropy

An action ${\displaystyle \alpha _{M}}$ of ${\displaystyle \mathbb {Z} ^{n}}$ by automorphisms of a compact metrizable abelian group may be associated via duality to any countable module ${\displaystyle N}$ over the ring ${\displaystyle R=\mathbb {Z} [z_{1}^{\pm 1},\dots ,z_{n}^{\pm 1}]}$.^[7] The topological entropy (which is equal to the measure-theoretic entropy) of this action, ${\displaystyle h(\alpha _{N})}$, is given by a Mahler measure (or is infinite).^[8] In the case of a cyclic module ${\displaystyle M=R/\langle F\rangle }$ for a non-zero polynomial ${\displaystyle F(z_{1},\dots ,z_{n})\in \mathbb {Z} [z_{1},\ldots ,z_{n}]}$ the formula proved by Lind, Schmidt, and Ward gives ${\displaystyle h(\alpha _{N})=\log M(F)}$, the logarithmic Mahler measure of ${\displaystyle F}$. In the general case, the entropy of the action is expressed as a sum of logarithmic Mahler measures over the generators of the principal associated prime ideals of the module. As pointed out earlier by Lind in the case ${\displaystyle n=1}$ of a single compact group automorphism, this means that the set of possible values of the entropy of such actions is either all of ${\displaystyle [0,\infty ]}$ or a countable set depending on the solution to Lehmer's problem. Lind also showed that the infinite-dimensional torus ${\displaystyle \mathbb {T} ^{\infty }}$ either has ergodic automorphisms of finite positive entropy or only has automorphisms of infinite entropy depending on the solution to Lehmer's problem.^[9]