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Università di Pisa
Sistema bibliotecario di ateneo
Tesi etd-11282008-095329
Condividi questa tesi:

Tipo di tesi Tesi di dottorato di ricerca
Autore PERUGINELLI, GIULIO
URN etd-11282008-095329
Titolo Integer values of polynomials
Settore scientifico disciplinare MAT/02 - ALGEBRA
Corso di studi MATEMATICA
Commissione
Nome Commissario Qualifica
Prof. Umberto Zannier Relatore
Parole chiave
• polynomial parametrization
• polynomial mapping
• integer-valued polynomial
• image of a polynomial
Data inizio appello 2008-12-13
Disponibilità unrestricted
Riassunto analitico
Let $f(X)$ be a polynomial with rational coefficients, $S$ be an
infinite subset of the rational numbers and consider the image set
$f(S)$. If $g(X)$ is a polynomial such that $f(S)=g(S)$ we say that
$g$ \emph{parametrizes} the set $f(S)$. Besides the obvious solution
$g=f$ we may want to impose some conditions on the polynomial $g$;
for example, if $f(S)\subset\Z$ we wonder if there exists a
polynomial with integer coefficients which parametrizes the set
$f(S)$.

Moreover, if the image set $f(S)$ is parametrized by a polynomial
$g$, there comes the question whether there are any relations
between the two polynomials $f$ and $g$. For example, if $h$ is a
linear polynomial and if we set $g=f\circ h$, the polynomial $g$
obviously parametrizes the set $f(\Q)$. Conversely, if we have
$f(\Q)=g(\Q)$ (or even $f(\Z)=g(\Z)$) then by Hilbert's
irreducibility theorem there exists a linear polynomial $h$ such
that $g=f\circ h$. Therefore, given a polynomial $g$ which
parametrizes a set $f(S)$, for an infinite subset $S$ of the
rational numbers, we wonder if there exists a polynomial $h$ such
that $f=g\circ h$. Some theorems by Kubota give a positive answer
under certain conditions.

The aim of this thesis is the study of some aspects of these two
problems related to the parametrization of image sets of
polynomials.

\bigskip

In the context of the first problem of parametrization we consider
the following situation: let $f$ be a polynomial with rational
coefficients such that it assumes integer values over the integers.
Does there exist a polynomial $g$ with integer coefficients such
that it has the same integer values of $f$ over the integers?

This kind of polynomials $f$ are called \emph{integer-valued}
polynomials. We remark that the set of integer-valued polynomials
strictly contains polynomials with integer coefficients: take for
example the polynomial $X(X-1)/2$, which is integer-valued over the
set of integers but it has no integer coefficients. So, if $f$ is an
integer-valued polynomial, we investigate whether the set $f(\Z)$
can be parametrized by a polynomial with integer coefficients; more
in general we look for a polynomial $g\in\Z[X_1,\ldots,X_m]$, for
some natural number $m\in\N$, such that $f(\Z)=g(\Z^m)$. In this
case we say that $f(\Z)$ is $\Z$-\emph{parametrizable}.

In a paper of Frisch and Vaserstein it is proved that the subset of
pythagorean triples of $\Z^3$ is parametrizable by a single triple
of integer-valued polynomials in four variables but it cannot be
parametrized by a single triple of integer coefficient polynomials
in any number of variables. In our work we show that there are
examples of subset of $\Z$ parametrized by an integer-valued
polynomial in one variable which cannot be parametrized by an
integer coefficient polynomial in any number of variables.

If $f(X)$ is an integer-valued polynomial, we give the following
characterization of the parametrization of the set $f(\Z)$: without
loss of generality we may suppose that $f(X)$ has the form $F(X)/N$,
where $F(X)$ is a polynomial with integer coefficients and $N$ is a
minimal positive integer. If there exists a prime $p$ different from
$2$ such that $p$ divides $N$ then $f(\Z)$ is not
$\Z$-parametrizable. If $N=2^n$ and $f(\Z)$ is $\Z$-parametrizable
then there exists a rational number $\beta$ which is the ratio of
two odd integers such that $f(X)=f(-X+\beta)$. Moreover
$f(\Z)=g(\Z)$ for some $g\in\Z[X]$ if and only if $f\in\Z[X]$ or
there exists an odd integer $b$ such that $f\in\Z[X(b-X)/2]$. We
show that there exists integer-valued polynomials $f(X)$ such that
$f(\Z)$ is $\Z$-parametrizable with a polynomial
$G(X_1,X_2)\in\Z[X_1,X_2]$, but $f(\Z)\not=g(\Z)$ for every
$g\in\Z[X]$.

\bigskip

In 1963 Schinzel gave the following conjecture: let $f(X,Y)$ be an
irreducible polynomial with rational coefficients and let $S$ be an
infinite subset of $\Q$ with the property that for each $x$ in $S$
there exists $y$ in $S$ such that $f(x,y)=0$; then either $f$ is
linear in $Y$ or $f$ is symmetric in the variables $X$ and $Y$.

We remark that if a curve is defined by a polynomial with Schinzel's
property then its genus is zero or one, since it contains infinite
rational points; here we use a theorem of Faltings which solved the
Mordell conjecture (if a curve has genus greater or equal to two
then the set of its rational points is finite). We will focus our
attention on the case of rational curves (genus zero). Our objective
is to describe polynomials $f(X,Y)$ with Schinzel's property whose
curve is rational and we give a conjecture which says that these
rational curves have a parametrization of the form
$(\varphi(T),\varphi(r(T)))$.

This problem is related to the main topic of parametrization of
image sets of polynomials in the following way: if
$(\varphi(T),\psi(T))$ is a parametrization of a curve $f(X,Y)=0$
(which means $f(\varphi(T),\psi(T))=0$), where $f$ is a polynomial
with Schinzel's property, let $S=\{\varphi(t)|t\in S'\}$ be the set
of the definition of Schinzel, where $S'\subset\Q$. Then for each
$t\in S'$ there exists $t'\in S'$ such that $\psi(t)=\varphi(t')$,
hence $\psi(S')\subset\varphi(S')$. So, in the case of rational
curves, the problem of Schinzel is related to the problem of
parametrization of rational values of rational functions with other
rational functions (we will show that under an additional hypothesis
we can assume that $(\varphi(T),\psi(T))$ are polynomials). In
particular, if $(\varphi(T),\psi(T))$ is a parametrization of a
curve defined by a symmetric polynomial, then
$\psi(T)=\varphi(a(T))$, where $a(T)$ is an involution (that is
$a\circ a=Id$). So in the case of rational symmetric plane curves we
have this classification in terms of the parametrization of the
curve.

It turns out that this argument is also related to Ritt's theory of
decomposition of polynomials. His work is a sort of "factorization"
of polynomials in terms of indecomposable polynomials, that is
non-linear polynomials $f$ such that there are no $g,h$ of degree
less than $\deg(f)$ such that $f=g\circ h$. The indecomposable
polynomials are some sort of "irreducible" elements of this kind of
factorization.

\vspace{1.5cm}

% 1o capitolo

In the first chapter we recall some basic facts about algebraic
function fields in one variable, the algebraic counterpart of
algebraic curves. In particular we state the famous Luroth's
theorem, which says that a non trivial subextension of a purely
trascendental field of degree one is purely trascendental.

We give the definition of minimal couple of rational functions that
we will use later to characterize algebraically a proper
parametrization of a rational curve. We conclude the chapter with
the general notion of valuation ring of a field and we characterize
valuation rings of a purely trascendental field in one variable
(which corresponds geometrically to the Riemann sphere, if for
example the base field is the field of the complex numbers).
Moreover valuation rings of algebraic function fields in one
variable are discrete valuation rings.

\bigskip

%2o capitolo

In the second chapter we state the first theorem of Ritt, which
deals with decomposition of polynomials with complex coefficients
with respect to the operation of composition. In a paper of 1922
Ritt proved out that two maximal decompositions (that is a
decomposition whose components are neither linear nor further
decomposable) of a complex polynomial have the same number of
components and their degrees are the same up to the order. We give a
proof in the spirit of the original paper of Ritt, which uses
concepts like monodromy groups of rational functions, coverings and
theory of blocks in the action of a group on a set.

%Several other proof of this theorem have been given after that,

This result can be applied in the case of an equation involving
compositions of polynomials: thanks to Ritt's theorem we know that
every side of the equation has the same number of indecomposable
component.

\bigskip

%3o capitolo

In the third chapter we give the classical definition of plane
algebraic curves, both in the affine and projective case. We show
that there is a bijection between the points of a non-singular curve
and the valuation rings of its rational function field (which are
called places of the curve). More generally speaking, if we have a
singular curve $C$, the set of valuation rings of its rational
function field is in bijection with the set of points of a
non-singular model $C'$ of the curve (that is the two curves $C$ and
$C'$ are birational), called desingularization of the curve.

Then we deal with curves whose points are parametrized by a couple
of rational functions in one parameter; we call these curves
rational. From a geometric point of view a rational curve has
desingularization which is a compact Riemann surface of genus zero,
thus isomorphic to $\mathbb{P}^1$. Finally we expose some properties
of parametrizations of rational curves; we show a simple criterium
which provides a necessary and sufficient condition that lets a
rational curve have a polynomial parametrization in terms of places
at infinity.

\bigskip

%4o capitolo

In the fourth chapter we study the aforementioned conjecture of
Schinzel.

For example, if $f(X,Y)=Y-a(X)$ then by taking $S$ the full set of
rational numbers we see that the couple $(f,S)$ satisfies the
Schinzel's property. If $f$ is symmetric and the set of rational
points of the curve determined by $f$ is infinite, then if we define
$S$ to be the projection on the first coordinate of the rational
points of the curve we obtain another example of polynomial with the
above property.

The hypothesis of irreducibility of the polynomial $f$ is required
because we want to avoid phenomenon such as $f(X,Y)=X^2-Y^2$ and
$S=\Q$, where $f$ is neither linear nor symmetric. In general if a
polynomial $f(X,Y)$ has $X-Y$ as a factor, then it admits the full
set of rational numbers as set $S$. Another example is the following
(private communication of Schinzel): let

$$f(X,Y)=(Y^2-XY-X^2-1)(Y^2-XY-X^2+1)$$

and $S=\{F_n\}_{n\in\N}$, where $F_n$ is the Fibonacci sequence
which satisfies the identity $F_{n+1}^2-F_{n+1}F_n-F_n^2=(-1)^n$ for
each natural number $n$; if $f_1,f_2\in\Q[X,Y]$ are the two
irreducible factors of $f$ then for each $n\in\N$ the couple of
integers $(F_n,F_{n+1})$ is a point of the curve associated to the
polynomial $f_1$ or $f_2$, according to the parity of $n$.

Zannier has recently given the following counterexample to
Schinzel's conjecture:

$$f(X,Y)=Y^2-2(X^2+X)Y+(X^2-X)^2$$

with $S$ equal to the set of rational (or integer) squares. The idea
is the following: it is well known that for each couple of rational
functions $(\varphi(t),\psi(t))$ with coefficients in a field $k$
there exists a polynomial $f\in k[X,Y]$ such that
$f(\varphi(t),\psi(t))=0$. In fact $k(t)$ has trascendental degree
one over $k$; we also say that $\varphi$ and $\psi$ are
algebraically dependent. Moreover if we require that the polynomial
$f$ is irreducible then it is unique up to multiplication by
constant.

This procedure allows us to build families of polynomials with
Schinzel's property: it is sufficient to take couples of rational
functions $(\varphi(t),\varphi(r(t)))$, where $\varphi(t),r(t)$ are
rational functions. If we consider the irreducible polynomial
$f\in\Q[X,Y]$ such that $f(\varphi(t),\varphi(r(t)))=0$ and the set
$S=\{\varphi(t)|t\in\Q\}$, we see that $(f,S)$ has Schinzel's
property. In particular Zannier's example is obtained from the
couple of rational functions $(\varphi(t),r(t))=(t^2,t(t+1))$. If
$\deg(\varphi)>1$ and $\deg(r(t))>1$ then it turns out that $f$ is
neither linear nor symmetric in $X$ and $Y$, but it is a polynomial
with Schinzel's property.

\bigskip

%5o capitolo

In the last chapter we deal with the problem of parametrization of
integer-valued polynomials and we prove the results mentioned at the
beginning of this introduction. The idea of the proof is the
following: let $f(X)=F(X)/N$ be an integer-valued polynomial as
above; since the set of integer-valued polynomials is a module over
$\Z$, we can assume that $N$ is a prime number $p$. We remark that a
bivariate polynomial of the form $f(X)-f(Y)$ has over $\Q$ only two
linear factors; moreover, the set of integer values $n$ such that
there exists $q\in\Q$ such that $(n,q)$ belongs to an irreducible
component of the curve $f(X)-f(Y)=0$ which is not linear in $Y$, has
zero density, by a theorem of Siegel. If $f(\Z)$ is
$\Z$-parametrizable by a polynomial
$g\in\Z[X_1,\ldots,X_m]=\Z[\underline{X}]$ then by Hilbert's
irreducibility theorem there exists $Q\in\Q[\underline{X}]$ such
that $F(Q(\underline{X}))=pg(\underline{X})$; we obtain necessary
conditions for such polynomial $Q$ in order to satisfy the previous
equality. In the same hypothesis, for each $n\in\Z$ there exists
$\underline{x}_n\in\Z^m$ such that $f(n)=f(Q(\underline{x}_n))$. So
we study how the points $(n,Q(\underline{x}_n))$, for $n\in\Z$,
distribute among the irreducible components of the curve
$f(X)-f(Y)=0$; by the aforementioned theorem of Siegel it turns out
that, up to a subset of density zero of $\Z$, they belong to
components determined by linear factors of $f(X)-f(Y)$. For each of
them, the projection on the first component of this kind of points
is a set of integers contained in a single residue class modulo the
prime $p$. So if $p$ is greater then two, which is the maximum
number of linear factors of a bivariate separated polynomial over
$\Q$, the set $f(\Z)$ is not $\Z$-parametrizable.

The problem of factorization of bivariate separated polynomials,
that is polynomials of the form $f(X)-g(Y)$, is a topic which has
been intensively studied for years (Cassels, Fried, Feit, Bilu,
Tichy, Zannier, Avanzi, Cassou-Noguès, Schinzel, etc...)

Our next aim is the classification of the integer-valued polynomials
$f(X)$ such that $f(\Z)$ is parametrizable with an integer
coefficient polynomial in more than one variable (for example
$f(X)=3X(3X-1)/2$). I conjecture that such polynomials (except when
$f\in\Z[X]$) belong to $\Z[p^kX(p^kX-a)/2]$, where $p$ is a prime
different from $2$, $a$ is an odd integer coprime with $p$ and $k$ a
positive integer. I show in my work that if $f(X)$ is such a
polynomial, then $f(\Z)$ is $\Z$-parametrizable.

Moreover we want to study the case of number fields, that is the
parametrization of sets $f(O_K)$, where $O_K$ is the ring of
integers of a number field $K$ and $f\in K[X]$ such that
$f(O_K)\subset O_K$, with polynomials with coefficients in the ring
$O_K$.
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