1. Introduction
A real algebraic integer
is called a
Pisot number after [
1,
2], if all the algebraic conjugates of
over the field of rational numbers
(other than
itself) are of absolute value
. Pisot numbers attract a lot of attention in the study of number expansions with algebraic number bases [
3,
4], substitution tilings [
5,
6,
7], integer sequences with particular regard to linear recurrences [
8,
9,
10], distributions of the fractional parts of the powers of real numbers [
11,
12] and many other areas [
13,
14].
Recently, there has been a surge in interest in complex-base number expansions [
15,
16,
17,
18]: in the distributions of the powers of algebraic numbers [
19,
20]; in the complex plane
with respect to the Gaussian lattice
; and in complex algebraic integers with special multiplicative properties [
21,
22,
23,
24]. In these kinds of problems, the complex analogues of the Pisot numbers in
play the same pivotal role as the Pisot numbers in
. Recall that an algebraic number
,
is called
a complex Pisot number if all of its algebraic conjugates
satisfy
. Complex Pisot numbers were considered first by Kelly and Samet [
25,
26]. The smallest complex Pisot numbers were identified by Chamfy [
27]; later, Garth [
28,
29] significantly expanded Chamfy’s list. Nonetheless, recent research has increased the general interest in the spectra of complex Pisot numbers.
In the present paper, we are interested in complex Pisot numbers that originate from the simplest possible polynomials, namely Borwein trinomials. If the polynomial
(1) has exactly three or four nonzero terms, then it is called
a trinomial or
a quadrinomial, respectively. The polynomials that have all their coefficients
are called Borwein polynomials (in honor of the late P. Borwein, as in [
30]). Thus, Borwein trinomials are polynomials of the form
. For example,
is a Borwein trinomial. The main result of our paper is Theorem 1.
Theorem 1. Any Borwein trinomial that has a complex Pisot number as its root is of the form , where is one of the 17 polynomials listed in Table 1. All the polynomials in
Table 1 are irreducible, except for
,
,
, and
, which are all divisible by
. In comparison, all Borwein trinomials and quadrinomials that give a rise to real Pisot numbers were essentially identified in [
31] (after taking into account the irreducibility theorem of Ljunggren [
32]). The proof of Theorem 1 is based on the following result.
Theorem 2. Let be positive integers. All Borwein trinomials with at most two roots inside the unit disc are given in Table 2. We also note that Borwein trinomials appear to have no multiple roots in (see Proposition 2).
More generally, the number of zeros of a Borwein trinomial or a Borwein quadrinomial is interesting in the context of the distribution of zeros of polynomials with small coefficients [
30]. For this, let us state the definition for the zero number
of a polynomial
. First, recall that
splits over the field of complex numbers
into
where the complex zeros
of
are not necessarily distinct. The zero counting functions with respect to the unit circle are introduced through the formulas
and
where the zeros are counted with the multiplicities. The
reciprocal polynomial
is defined by
Hence, one always has that
A complex number with absolute value 1 is called a unimodular number. Note that every root of unity is a unimodular number. However, not every unimodular number is a root of unity, since for every positive integer n.
We derive Theorem 2 from Proposition 3, which gives explicit formulas for for any Borwein trinomial . Finally, Proposition 3 is derived from an old result of Bohl (see Theorem 3).
Previous work on the smallest complex Pisot numbers [
27,
28,
29] was based on the complicated computation of the coefficients of Taylor–Maclaurin series of bounded analytic functions (Schur functions), a method pioneered by Dufresnoy and Pisot [
1,
2]. Our new contribution to expand the list of known complex Pisot numbers is based on Bohl’s formula [
33,
34] (discussed below in the next section).
The paper is organized as follows. In
Section 2, we prove Proposition 3 and Theorems 1 and 2. The irreducibility of Borwein trinomials is considered in
Section 3. We explicitly describe irreducible Borwein trinomials (see Corollary 4). This result has already been proven by Ljunggren (see Theorem 3 in [
35]). Nevertheless, we give an alternative proof based on Proposition 3.
2. Proofs of Theorems
Let x be a real number. Recall that denotes the largest rational integer that is less than or equal to x. Similarly, denotes the smallest rational integer that is greater than or equal to x. We will need the following basic properties of and , which follow directly from the definitions of these functions.
Proposition 1. The following statements are true.
- (i)
For any real number x, .
- (ii)
For any real number x, the equalities and hold.
- (iii)
For any real numbers a and b, , the interval contains exactly rational integers.
The main tool in the proof of Theorem 2 is the following result due to Bohl: for a modern formulation, see the expository note [
34] (also formulated as Theorem 3.2 in [
36]).
Theorem 3 (Bohl’s theorem, [
33,
36])
. Let be a trinomial, where and m and n are coprime positive integers such that . Assume that, for a real number , there exists a triangle with edge lengths , , and . Let and . Then, the number of roots of that lie in the open disc is given by the number of integers located in the open interval , whereand Note that if is a polynomial such that and ℓ is a positive integer, then .
Proposition 2. Let be positive integers and . Then, the polynomial has no multiple roots in .
Proof of Proposition 2. For a contradiction, assume that is a multiple root of . Then, . Since , we have that and . Hence, . On the other hand, implies that . Substituting this into yields . Hence, and . Therefore, , which contradicts the previously obtained inequality . □
Proposition 3. Let be coprime positive integers. Then, Proof of Proposition 3. We will apply Theorem 3 to the polynomial
, where
,
, and
. Note that, in Theorem 3, the triangle with edge lengths
,
, and
is an equilateral triangle. Hence,
and
. By Theorem 3,
equals the number of integers located in the open interval
, where
We will consider only the case . The remaining formulas for in Proposition 3 can be obtained completely analogously.
Let
. Then,
and
. Hence,
. By Theorem 3,
equals the number of integers located in the open interval
. Hence, in view of Proposition 1 (iii),
Note that
. Then, in view of Proposition 1 (ii),
□
The reciprocal polynomial is of the form . Therefore, Proposition 3 implies the following corollary.
Corollary 1. Let be coprime positive integers. Then, Note that for
,
equals the number of roots of
that lie strictly outside the unit circle
. Now, in view of Proposition 3, Corollary 1, and the formula
we can determine the number of unimodular roots of
.
Corollary 2. Let be coprime positive integers. Then, Proof of Corollary 2. We will consider only the case . The remaining formulas for can be obtained completely analogously.
Let
. By Proposition 3 and Corollary 1,
Hence, the formula
implies
□
The following corollary has already been proven by Ljunggren (see Theorem 3 in [
35]). Nevertheless, we give an alternative proof of this result.
Corollary 3. Let be coprime positive integers.
- 1.
The polynomial has a unimodular root if and only if is divisible by 3. Furthermore, if is divisible by 3, then , where the polynomial has no unimodular roots.
- 2.
The polynomial has a unimodular root if and only if is divisible by 6. Furthermore, if is divisible by 6, then , where the polynomial has no unimodular roots.
- 3.
The polynomial has a unimodular root if and only if is divisible by 6. Furthermore, if is divisible by 6, then , where the polynomial has no unimodular roots.
- 4.
The polynomial has a unimodular root if and only if is divisible by 6. Furthermore, if is divisible by 6, then , where the polynomial has no unimodular roots.
Note that the polynomial in this corollary is irreducible (see Theorem 4).
Proof of Corollary 3. The first part of every proposition follows directly from Corollary 2.
1. Assume that
is divisible by 3. According to Corollary 2, the trinomial
has precisely two unimodular roots. It suffices to show that
is a root of
(indeed, if
, then
, so that
and
are the only unimodular roots of
and
divides
). We have that
for some positive integer
t. Moreover,
is a primitive third root of unity, whose minimal polynomial is
. Since
, we have that
Note that m is not divisible by 3 since m and n are coprime and . Hence, is also a primitive third root of unity, and thus a root of . Therefore, .
2. Assume that
is divisible by 6. According to Corollary 2, the trinomial
has precisely two unimodular roots. As in the proof of the first proposition, it suffices to show that
is a root of
. We have that
for some positive integer
t. Moreover,
is a primitive sixth root of unity, whose minimal polynomial is
. Since
, we have that
Note that m is coprime to 6 since m and n are coprime and . Hence, is also a primitive sixth root of unity, and thus a root of . Therefore, .
3. Assume that
is divisible by 6. According to Corollary 2, the trinomial
has precisely two unimodular roots. As in the proof of the first proposition, it suffices to show that
is a root of
. We have that
for some positive integer
t. Moreover,
is a primitive sixth root of unity, whose minimal polynomial is
. Since
, we have that
Note that m is coprime to 6 since m and n are coprime and . Hence, is also a primitive sixth root of unity, and thus a root of . Therefore, .
4. This proposition follows from the second proposition by considering the reciprocal polynomial . □
Proof of Theorem 2. Let be positive integers. Suppose that is a Borwein trinomial such that . Consider two possible cases: gcd and gcd.
Case 1. We have that gcd
. We will apply Theorem 3 to the polynomial
, where
,
and
. Note that in Theorem 3, the triangle with edge lengths
,
, and
is an equilateral triangle. Hence,
and
. By Theorem 3,
equals the number of integers located in the open interval
, where
Hence, by (iii) and (i) of Proposition 1, we have
Recall that
. Thus,
, which is equivalent to
. Thus, we are left to compute
for every polynomial
, where
, gcd
, and
. In total, there are 13 pairs
satisfying these conditions, namely
Hence, there are exactly
polynomials
to be considered. Applying Proposition 3 (one can use any mathematics software, e.g., SageMath [
37]), we obtain all such polynomials with
and
, which are given in the first and third columns of
Table 2, respectively.
Case 2. We have that gcd
. Denote
. Then
and
for some coprime positive integers
. Furthermore,
, where
is a Borwein trinomial. One has that
. This, in view of
and
, implies
,
, and
. We have already determined all Borwein trinomials
with
in Case 1 (see the first column in
Table 2). Hence,
for any polynomial
from the first column of
Table 2. All such trinomials
with
are given in the second column of
Table 2. □
Proof of Theorem 1. Let
be a Borwein trinomial such that one of its roots, say
, is a complex Pisot number. Denote by
the minimal polynomial of
. Then,
is irreducible and divides
. By Theorem 4, every root (if any) of the quotient
is a unimodular number (if
is irreducible, then
and
). Hence, both polynomials
and
have the same number of roots outside the unit circle
, and this number equals 2 since
is the minimal polynomial of a complex Pisot number. Therefore,
. Now, we have that the Borwein trinomial
has exactly two roots inside the unit circle
, namely
and
. Recall that
. Thus, both roots of
inside the unit circle
are non-real numbers. On the other hand,
Table 2 lists all Borwein trinomials
with
in the second and third columns (see Theorem 2). One can easily check that all of these polynomials have two non-real roots inside the unit circle
, except for polynomials
,
,
,
, and
, which all have two real roots inside the unit circle
. Hence, all Borwein trinomials
, which have a complex Pisot number as a root, are given in
Table 1. □
3. Irreducibility of Borwein Trinomials
Selmer [
38] studied the irreducibility of trinomials
. In particular, he proved that the trinomial
is irreducible for every positive integer
. Tverberg [
39] proved that a trinomial
is reducible if and only if it has a unimodular root. Ljunggren [
35] extended this result to any quadrinomial
.
Theorem 4 ([
39] and Theorem 3 in [
35])
. Let be positive integers. The trinomial is reducible over the rationals if and only if it has a unimodular root. If has unimodular roots, these roots can be collected to give a rational factor of . The other factor of is then irreducible. Note that, for any polynomial and any positive integer a, one has that . Thus, has a unimodular root if and only if has a unimodular root. Combining this and Theorem 4, we obtain that, for any positive integer a, the trinomial is irreducible if and only if the trinomial is irreducible. Hence, considering the irreducibility of a trinomial , one can always assume that m and n are coprime.
The following corollary has already been proven by Ljunggren (see Theorem 3 in [
35]). Nevertheless, we give an alternative proof of this result.
Corollary 4. Let be positive integers and .
- 1.
The polynomial is reducible if and only if is divisible by 3. Furthermore, if is divisible by 3, then has exactly unimodular roots, which are the roots of , and the quotient is an irreducible polynomial.
- 2.
The polynomial is reducible if and only if is divisible by 6. Furthermore, if is divisible by 6, then has exactly unimodular roots, which are the roots of , and the quotient is an irreducible polynomial.
- 3.
The polynomial is reducible if and only if is divisible by 6. Furthermore, if is divisible by 6, then has exactly unimodular roots, which are the roots of , and the quotient is an irreducible polynomial.
- 4.
The polynomial is reducible if and only if is divisible by 6. Furthermore, if is divisible by 6, then has exactly unimodular roots, which are the roots of , and the quotient is an irreducible polynomial.
Proof of Corollary 4. We will consider only the case . The remaining three propositions can be proven completely analogously.
Let and . Note that and are coprime and . Furthermore, . Hence, by Theorem 4, the trinomial is reducible if and only if the trinomial has a unimodular root. By Corollary 3, the trinomial has a unimodular root if and only if is divisible by 3. This proves the first part of the proposition.
Assume that is divisible by 3. Then, by Corollary 3, the trinomial can be factored as , where is a polynomial that has no unimodular roots. Hence, . Note that every root of is a root of unity since . Finally, Theorem 4 implies that the quotient is an irreducible polynomial. □
A real algebraic integer
is called a
Salem number after [
40,
41,
42], if all other algebraic conjugates of
lie in the unit disc
with at least one conjugate on the unit circle
. In particular, the minimal polynomial of every Salem number is of even degree and self-reciprocal:
. Note that none of the algebraic conjugates of a Salem number is a root of unity. Therefore, by Corollary 4, no Salem number is the root of a Borwein trinomial.