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### arXiv:1810.12413v1 [math.NT] 29 Oct 2018

LOWER BOUNDS FOR MAHLER MEASURE THAT DEPEND ON THE NUMBER OF MONOMIALS

SHABNAM AKHTARI AND JEFFREY D. VAALER

Abstract. We prove a new lower bound for the Mahler measure of a polyno-mial in one and in several variables that depends on the complex coefficients, and the number of monomials. In one variable our result generalizes a classical inequality of Mahler. In M variables our result depends on ZM as an ordered

group, and in general our lower bound depends on the choice of ordering.

1. Introduction

Let P (z) be a polynomial in C[z] that is not identically zero. We assume to begin with that P has degree N , and that P factors into linear factors in C[z] as

(1.1) P (z) = c0+ c1z + c2z2+ · · · + cNzN = cN N

Y

n=1

(z − αn).

If e : R/Z → T is the continuous isomorphism given by e(t) = e2πit, then the

Mahler measure of P is the positive real number

(1.2) M(P ) = exp Z R/Z log P e(t) dt  = |cN| N Y n=1 max{1, |αn|}.

The equality on the right of (1.2) follows from Jensen’s formula. If P1(z) and P2(z)

are both nonzero polynomials in C[z], then it is immediate from (1.2) that M P1P2= M P1M P2.

Mahler measure plays an important role in number theory and in algebraic dynam-ics, as discussed in [6], [12], [14, Chapter 5], and [16]. Here we restrict our attention to the problem of proving a lower bound for M(P ) when the polynomial P (z) has complex coefficients. We establish an analogous result for polynomials in several variables.

For P (z) of degree N and given by (1.1), there is a well known lower bound due to Mahler which asserts that

(1.3) |cn| ≤  N n  M(P ), for each n = 0, 1, 2, . . . , N .

The inequality (1.3) is implicit in [9], and is stated explicitly in [11, section 2], (see also the proof in [1, Theorem 1.6.7]). If

P (z) = (z ± 1)N,

2010 Mathematics Subject Classification. 11R06.

Key words and phrases. Mahler Measure, polynomial inequalities. Shabnam Akhtari’s research is funded by the NSF grant DMS-1601837.

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then there is equality in (1.3) for each n = 0, 1, 2, . . . , N .

We now assume that P (z) is a polynomial in C[z] that is not identically zero, and we assume that P (z) is given by

(1.4) P (z) = c0zm0+ c1zm1+ c2zm2+ · · · + cNzmN,

where N is a nonnegative integer, and m0, m1, m2, . . . , mN, are nonnegative integers

such that

(1.5) m0< m1< m2< · · · < mN.

We wish to establish a lower bound for M(P ) which depends on the coefficients and on the number of monomials, but which does not depend on the degree of P . Such a result was recently proved by Dobrowolski and Smyth [5]. We use a similar argument, but we obtain a sharper result that includes Mahler’s inequality (1.3) as a special case.

Theorem 1.1. Let P (z) be a polynomial in C[z] that is not identically zero, and is given by(1.4). Then we have

(1.6) |cn| ≤  N n  M(P ), for each n = 0, 1, 2, . . . , N .

Let f : R/Z → C be a trigonometric polynomial, not identically zero, and a sum of at most N + 1 distinct characters. Then we can write f as

(1.7) f (t) =

N

X

n=0

cne(mnt),

where c0, c1, c2, . . . , cN, are complex coefficients, and m0, m1, m2, . . . , mN, are

in-tegers such that

m0< m1< m2< · · · < mN.

As f is not identically zero, the Mahler measure of f is the positive number

M(f ) = exp Z R/Z log |f (t)| dt  .

It is trivial that f (t) and e(−m0t)f (t) have the same Mahler measure. Thus we get

the following alternative formulation of Theorem 1.1.

Corollary 1.1. Let f (t) be a trigonometric polynomial with complex coefficients that is not identically zero, and is given by(1.7). Then we have

(1.8) |cn| ≤  N n  M(f ), for eachn = 0, 1, 2, . . . , N .

For positive integers M we will prove an extension of Corollary 1.1 to trigono-metric polynomials

(1.9) F : (R/Z)M → C,

that are not identically zero. The Fourier transform of F is the function b

F : ZM → C, defined at each lattice point k in ZM by

(1.10) F (k) =b

Z

(R/Z)M

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In the integral on the right of (1.10) we write dx for integration with respect to a Haar measure on the Borel subsets of (R/Z)M normalized so that (R/Z)M has

measure 1. We write k for a (column) vector in ZM, kT for the transpose of k, x

for a (column) vector in (R/Z)M, and therefore

kTx= k1x1+ k2x2+ · · · + kNxN.

As F is not identically zero, the Mahler measure of F is the positive real number

M(F ) = exp Z (R/Z)M log F (x) dx  .

We assume that S ⊆ ZM is a nonempty, finite set that contains the support of bF .

That is, we assume that

(1.11) {k ∈ ZM : bF (k) 6= 0} ⊆ S,

and therefore F has the representation

(1.12) F (x) = X

k∈S

b

F (k)e kTx.

Basic results in this setting can be found in Rudin [13, Sections 8.3 and 8.4]. If α = (αm) is a (column) vector in RM, we write

ϕα: ZM → R

for the homomorphism given by

(1.13) ϕα(k) = kTα= k1α1+ k2α2+ · · · + kMαM.

It is easy to verify that ϕαis an injective homomorphism if and only if the

coordi-nates α1, α2, . . . , αM, are Q-linearly independent real numbers.

Let the nonempty, finite set S ⊆ ZM have cardinality N + 1, where 0 ≤ N . If

ϕα is an injective homomorphism, then the set



ϕα(k) : k ∈ S

consists of exactly N + 1 real numbers. It follows that the set S can be indexed so that

(1.14) S=k0, k1, k2, . . . , kN , and

(1.15) ϕα k0< ϕα k1< ϕα k2< · · · < ϕα kN.

By using a limiting argument introduced in a paper of Boyd [2], we will prove the following generalization of (1.8).

Theorem 1.2. Let F : (R/Z)M → C be a trigonometric polynomial that is not

identically zero, and is given by (1.12). Let ϕα : ZM → R be an injective

homo-morphism, and assume that the finite set S, which contains the support of bF , is indexed so that(1.14) and (1.15) hold. Then we have

(1.16) bF kn ≤  N n  M(F ), for each n = 0, 1, 2, . . . , N .

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Let F and ϕα : ZM → R be as in the statement of Theorem 1.2, and then

let ϕβ : ZM → R be a second injective homomorphism. It follows that S can be

indexed so that (1.14) and (1.15) hold, and S can also be indexed so that (1.17) S=ℓ0, ℓ1, ℓ2, . . . , ℓN ,

and

(1.18) ϕβ ℓ0< ϕβ ℓ1< ϕβ ℓ2< · · · < ϕβ ℓN.

In general the indexing (1.14) is distinct from the indexing (1.17). Therefore the system of inequalities (1.19) bF kn ≤  N n  M(F ), for each n = 0, 1, 2, . . . , N , and (1.20) bF ℓn ≤  N n  M(F ), for each n = 0, 1, 2, . . . , N ,

which follow from Theorem 1.2, are different, and in general neither system of inequalities implies the other.

2. Proof of Theorem 1.1

It follows from (1.2) that the polynomial P (z), and the polynomial z−m0P (z),

have the same Mahler measure. Hence we may assume without loss of generality that the exponents m0, m1, m2, . . . , mN, in the representation (1.4) satisfy the more

restrictive condition

(2.1) 0 = m0< m1< m2< · · · < mN.

If N = 0 then (1.6) is trivial. If N = 1, then  1 0  =  1 1  = 1,

and using Jensen’s formula we find that M c0+ c1zm1

= max{|c0|, |c1|}.

Therefore the inequality (1.6) holds if N = 1. Throughout the remainder of the proof we assume that 2 ≤ N , and we argue by induction on N . Thus we assume that the inequality (1.6) holds for polynomials that can be expressed as a sum of strictly less than N + 1 monomials.

Besides the polynomial

(2.2) P (z) = c0zm0+ c1zm1+ c2zm2+ · · · + cNzmN,

we will work with the polynomial (2.3) Q(z) = zmNP z−1= c

0zmN−m0+ c1zmN−m1+ c2zmN−m2+ · · · + cN.

It follows from (1.2) that

(2.4) M(Q) = exp

Z

R/Z

log e(mNt)P e(−t) dt



= M(P ).

Next we apply an inequality of Mahler [10] to conclude that both

(2.5) M P′

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Because P′(z) = N X n=1 cnmnzmn−1

is a sum of strictly less than N +1 monomials, we can apply the inductive hypothesis to P′. It follows that (2.6) |cn|mn ≤  N − 1 n − 1  M P′ ≤ mN  N − 1 n − 1  M(P ) for each n = 1, 2, . . . , N . As m0= 0, and  N − 1 −1  = 0,

it is trivial that (2.6) also holds at n = 0. In a similar manner, Q′ (z) = N −1X n=0 cn(mN− mn)zmN−mn−1

is a sum of strictly less that N + 1 monomials. We apply the inductive hypothesis to Q′, and get the inequality

(2.7) |cn|(mN − mn) ≤  N − 1 N − 1 − n  M Q′ ≤ mN  N − 1 n  M(Q)

for each n = 0, 1, 2, . . . , N − 1. In this case we have

(mN − mN) = 0, and  N − 1 N  = 0,

and therefore (2.7) also holds at n = N .

To complete the proof we use the identity (2.4), and we apply the inequality (2.6), and the inequality (2.7). In this way we obtain the bound

|cn|mN = |cn|mn+ |cn|(mN − mn) ≤ mN  N − 1 n − 1  M(P ) + mN  N − 1 n  M(P ) = mN  N n  M(P ). (2.8) This verifies (1.6).

3. Archimedean orderings in the group ZM

In this section we consider ZM as an ordered group. To avoid degenerate situa-tions, we assume throughout this section that 2 ≤ M .

Let α belong to RM, and let ϕ

α : ZM → R be the homomorphism defined by

(1.13). We assume that the coordinates α1, α2, . . . , αM, are Q-linearly independent

so that ϕα is an injective homomorphism. It follows, as in [13, Theorem 8.1.2 (c)],

that ϕαinduces an archimedean ordering in the group ZM. That is, if k and ℓ are

distinct points in ZM we write k < ℓ if and only if

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in R. Therefore ZM, < is an ordered group, and the order is archimedean. If

S⊆ ZM is a nonempty, finite subset of cardinality N + 1, then the elements of S can be indexed so that

(3.1) S=k0, k1, k2, . . . , kN and (3.2) kT 0α< k T 1α< k T 2α< · · · < k T Nα.

A more general discussion of ordered groups is given in [13, Chapter 8]. Here we require only the indexing (3.1) that is induced in the finite subset S by the injective homomorphism ϕα.

If b = (bm) is a (column) vector in ZM, we define the norm

(3.3) kbk∞= max



|bm| : 1 ≤ m ≤ M

. And if S ⊆ ZM is a nonempty, finite subset we write

kSk∞= max



kkk∞: k ∈ S

. Following Boyd [2], we define the function

ν : ZM\ {0} → {1, 2, 3, . . . } by

(3.4) ν(a) = minkbk∞: b ∈ ZM, b 6= 0, and b Ta

= 0 .

It is known (see [2]) that the function a 7→ ν(a) is unbounded, and a stronger conclusion follows from our Lemma 3.2. Moreover, if ν(a) is sufficiently large, then the map k 7→ kTarestricted to points k in the finite subset S takes distinct integer values, and therefore induces an ordering in S. This follows immediately from the triangle inequality for the norm (3.3), and was noted in [2]. As this result will be important in our proof of Theorem 1.2, we prove it here as a separate lemma. Lemma 3.1. [D. Boyd] Let S ⊆ ZM be a nonempty, finite subset with cardinality

|S| = N + 1, and let a 6= 0 be a point in ZM such that

(3.5) 2kSk∞< ν(a).

Then

(3.6) kTa: k ∈ S

is a collection of N + 1 distinct integers.

Proof. If N = 0 the result is trivial. Assume that 1 ≤ N , and let k and ℓ be distinct points in S. If kTa= ℓTa, then (k − ℓ)Ta= 0. It follows that ν(a) ≤ kk − ℓk∞≤ kkk∞+ kℓk∞≤ 2kSk∞,

and this contradicts the hypothesis (3.5). We conclude that (3.6) contains N + 1

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Let ϕα : ZM → R be an injective homomorphism, and let S ⊆ ZM be a

nonempty, finite subset of cardinality N + 1. We assume that the elements of S are indexed so that both (3.1) and (3.2) hold. If a 6= 0 in ZM satisfies (3.5), then

it may happen that the indexing (3.1) also satisfies the system of inequalities kT0a< kT1a< kT2a< · · · < kTNa.

We write B(α, S) for the collection of such lattice points a. That is, we define B(α, S) =a∈ ZM : 2kSk∞< ν(a)

and kT0a< kT1a< kT2a< · · · < kTNa

. (3.7)

The following lemma establishes a crucial property of B(α, S).

Lemma 3.2. Let the subset B(α, S) be defined by (3.7). Then B(α, S) is an infinite set, and the functionν restricted to B(α, S), is unbounded on B(α, S). Proof. By hypothesis (3.8) η = η(α, S) = minkT nα− k T n−1α: 1 ≤ n ≤ N is a positive constant that depends on α and S.

By Dirichlet’s theorem in Diophantine approximation (see [3] or [15]), for each positive integer Q there exists an integer q such that 1 ≤ q ≤ Q, and

(3.9) maxkqαmk : m = 1, 2, . . . , M ≤ (Q + 1)−1 M ≤ (q + 1)− 1 M,

where k k on the left of (3.9) is the distance to the nearest integer function. Let Q be the collection of positive integers q such that

(3.10) maxkqαmk : m = 1, 2, . . . , M

≤ (q + 1)−1 M.

Because 2 ≤ M , at least one of the coordinates αmis irrational, and it follows from

(3.9) that Q is an infinite set.

For each positive integer q in Q, we select integers b1q, b2q, . . . , bMq, so that

(3.11) kqαmk = |qαm− bmq|, for m = 1, 2, . . . , M .

Then (3.10) can be written as

(3.12) max|qαm− bmq| : m = 1, 2, . . . , M ≤ (q + 1)−1 M. Let bq = bmq 

be the corresponding lattice point in ZM, so that q 7→ b

q is a map

from Q into ZM. It follows using (3.8) and (3.12), that for each index n we have

qη ≤ qkTnα− qk T n−1α = kTnbq− kTn−1bq+ kn− kn−1 T (qα − bq) ≤ kTnbq− kTn−1bq+ 2kSk∞ XM m=1 |qαm− bmq|  ≤ kTnbq− kTn−1bq+ 2kSk∞M (q + 1) −1 M.

Therefore for each sufficiently large integer q in Q, the lattice point bq satisfies the

system of inequalities

kT0bq < kT1bq< kT2bq < · · · < kTNbq. We conclude that for a sufficiently large integer L we have (3.13) bq: L ≤ q and q ∈ Q ⊆ B(α, S).

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This shows that B(α, S) is an infinite set.

To complete the proof we will show that the function ν is unbounded on the infinite collection of lattice points

(3.14) bq : L ≤ q and q ∈ Q .

If ν is bounded on (3.14), then there exists a positive integer B such that

(3.15) ν(bq) ≤ B

for all points bq in the set (3.14). Let CB be the finite set

CB=



c∈ ZM : 1 ≤ kck≤ B .

Because α1, α2, . . . , αM, are Q-linearly independent, and CBis a finite set of nonzero

lattice points, we have

(3.16) 0 < δB = min  M X m=1 cmαm :c∈ CB  .

By our assumption (3.15), for each point bqin (3.14) there exists a point cq = (cmq)

in CB, such that (3.17) cTqbq= M X m=1 cmqbmq= 0.

Using (3.12) and (3.17), we find that

qδB ≤ q M X m=1 cmqαm = M X m=1 cmq qαm− bmq  ≤ XM m=1 |cmq|  max|qαm− bmq| : m = 1, 2, . . . , M ≤ M B(q + 1)−1 M. (3.18)

But (3.18) is impossible when q is sufficiently large, and the contradiction implies that the assumption (3.15) is false. We have shown that ν is unbounded on the set (3.14). In view of (3.13), the function ν is unbounded on B(α, S). 

4. Proof of Theorem 1.2

If M = 1 then the inequality (1.16) follows from Corollary 1.1. Therefore we assume that 2 ≤ M .

Let ϕα : ZM → R be an injective homomorphism, and let the set S be indexed

so that (1.14) and (1.15) hold. It follows from Lemma 3.2 that the collection of lattice points B(α, S) defined by (3.7), is an infinite set, and the function ν defined by (3.4) is unbounded on B(α, S).

Let a be a lattice point in B(α, S). If F : (R/Z)M → C is given by (1.12), we

define an associated trigonometric polynomial Fa: R/Z → C in one variable by

(4.1) Fa(t) = X k∈S b F (k)e kTat= N X n=0 b F (kn)e kTnat  ,

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where the equality on the right of (4.1) uses the indexing (1.14) induced by ϕα. The

hypothesis (1.15) implies that the integer exponents on the right of (4.1) satisfy the system of inequalities (4.2) kT 0a< k T 1a< k T 2a< · · · < k T Na.

Then it follows from (1.8), (4.1), and (4.2), that

(4.3) bF kn ≤  N n  M(Fa), for each n = 0, 1, 2, . . . , N .

We have proved that the system of inequalities (4.3) holds for each lattice point a in B(α, S).

To complete the proof we appeal to an inequality of Boyd [2, Lemma 2], which asserts that if b is a parameter in ZM then

(4.4) lim sup

ν(b)→∞

M Fb≤ M(F ).

More precisely, if b1, b2, b3, . . . , is a sequence of points in ZM such that

(4.5) lim j→∞ν(bj) = ∞, then (4.6) lim sup j→∞ M Fb j  ≤ M(F ).

Because ν is unbounded on B(α, S), there exists a sequence b1, b2, b3, . . . , contained

in B(α, S) that satisfies (4.5). Hence the sequence b1, b2, b3, . . . , in B(α, S) also

satisfies (4.6). From (4.3) we have

(4.7) bF kn ≤  N n  M(Fb j),

for each n = 0, 1, 2, . . . , N , and for each j = 1, 2, 3, . . . . The inequality (1.16) plainly follows from (4.6) and (4.7). This completes the proof of Theorem 1.2.

Boyd conjectured in [2] that (4.4) could be improved to

(4.8) lim

ν(b)→∞

M Fb= M(F ).

The proposed identity (4.8) was later verified by Lawton [8] (see also [4] and [7]). Here we have used Boyd’s inequality (4.4) because it is simpler to prove than (4.8), and the more precise result (4.8) does not effect the inequality (1.16).

References

[1] E. Bombieri and W. Gubler, Heights in Diophantine Geometry, Cambridge U. Press, New York, 2006.

[2] D. W. Boyd, Kronecker’s Theorem and Lehmer’s Problem for Polynomials in Several Variables, Journal of Number Theory13, (1981), 116–121.

[3] J. W. S. Cassels, An Introduction to Diophantine Approximation, Cambridge U. Press, Lon-don, 1965.

[4] E. Dobrowolski, A note on Lawton’s theorem, Canad. Math. Bull. 60, no. 3, (2017), 484–489. [5] E. Dobrowolski and C. Smyth, Mahler measures of polynomials that are sums of a bounded number of monomials, International Journal of Number Theory, 13, No. 6 (2017), 1603–1610. [6] G. Everest and T. Ward, Heights of Polynomials and Entropy in Algebraic Dynamics,

Springer-Verlag, London, 1999.

[7] Z. Issa and M. Lal´ın, A Generalization of a Theorem of Boyd and Lawton, Canad. Math. Bull. 56, no. 4, (2013), 759–768.

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[8] W. M. Lawton, A Problem of Boyd concerning Geometric Means of Polynomials, Journal of Number Theory, 16 (1983), 356–362.

[9] K. Mahler, An application of Jensen’s formula to polynomials, Mathematika, 7 (1960), 98–100. [10] K. Mahler, On the zeros of the derivative of a polynomial, Proceedings of the Royal Society,

Ser. A, (London) Vol. 264, (1961), 145–154.

[11] K. Mahler, Some inequalities for polynomials in several variables, Journal London Math. Soc.,37 (1962), 341–344.

[12] I. E. Pritsker, Mahler’s measure, and multipliers, in Number Theory and Polynomials, ed. J. McKee and C. Smyth, London Math. Soc. Lecture Notes Series, 352, 255–276.

[13] W. Rudin, Fourier Analysis on Groups Interscience Pub., New York, (1962). [14] K. Schmidt, Dynamical Systems of Algebraic Origin, Birkh¨auser Verlag, Basel, 1995. [15] W. M. Schmidt, Diophantine Approximation, Lecture Notes in Mathematics 785,

Springer-Verlag, Berlin, 1980.

[16] C. Smyth, The Mahler measure of algebraic numbers: a survey, in Number Theory and Polynomials, ed. J. McKee and C. Smyth, London Math. Soc. Lecture Notes Series, 352, 322–349.

Department of Mathematics, University of Oregon, Eugene, Oregon 97402 USA, Max Planck Institute for Mathematics, Vivatsgasse 7, 53111 Bonn, Germany

Department of Mathematics, University of Texas, Austin, Texas 78712 USA E-mail address: vaaler@math.utexas.edu

References

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