<p>Rational transformations play an important role in the construction of irreducible polynomials over finite fields. Usually, the methods involve fixing a rational function <i>Q</i> and deriving conditions on polynomials <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10623_2025_1591_Article_IEq1.gif" Format="GIF" Height="20" Rendition="HTML" Resolution="72" Type="Linedraw" Width="70" /> </InlineMediaObject> <EquationSource Format="TEX">\(F\in \mathbb {F}_q[x]\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>F</mi> <mo>∈</mo> <msub> <mi mathvariant="double-struck">F</mi> <mi>q</mi> </msub> <mrow> <mo stretchy="false">[</mo> <mi>x</mi> <mo stretchy="false">]</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> such that the rational transformation of <i>F</i> with <i>Q</i> is irreducible. Here we want to change the perspective and study rational functions with which the rational transformation never yields irreducible polynomials. We show that if the rational function is contained in certain subfields of <InlineEquation ID="IEq2"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10623_2025_1591_Article_IEq2.gif" Format="GIF" Height="20" Rendition="HTML" Resolution="72" Type="Linedraw" Width="40" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathbb {F}_q(x)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi mathvariant="double-struck">F</mi> <mi>q</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> then the rational transformation with it is always reducible. This extends the list of known examples.</p>

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Rational transformations over finite fields that are never irreducible

  • Max Schulz

摘要

Rational transformations play an important role in the construction of irreducible polynomials over finite fields. Usually, the methods involve fixing a rational function Q and deriving conditions on polynomials \(F\in \mathbb {F}_q[x]\) F F q [ x ] such that the rational transformation of F with Q is irreducible. Here we want to change the perspective and study rational functions with which the rational transformation never yields irreducible polynomials. We show that if the rational function is contained in certain subfields of \(\mathbb {F}_q(x)\) F q ( x ) then the rational transformation with it is always reducible. This extends the list of known examples.