<p>We consider the series <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="9_2025_2926_Article_IEq1.gif" Format="GIF" Height="20" Rendition="HTML" Resolution="72" Type="Linedraw" Width="136" /> </InlineMediaObject> <EquationSource Format="TEX">\(\sum _{n=1}^{\infty } z^{n} (a_{n} + x)^{-s}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msubsup> <mo>∑</mo> <mrow> <mi>n</mi> <mo>=</mo> <mn>1</mn> </mrow> <mi>∞</mi> </msubsup> <msup> <mi>z</mi> <mi>n</mi> </msup> <msup> <mrow> <mo stretchy="false">(</mo> <msub> <mi>a</mi> <mi>n</mi> </msub> <mo>+</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> <mrow> <mo>-</mo> <mi>s</mi> </mrow> </msup> </mrow> </math></EquationSource> </InlineEquation> where <InlineEquation ID="IEq2"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="9_2025_2926_Article_IEq2.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="40" /> </InlineMediaObject> <EquationSource Format="TEX">\(\{a_{n}\}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">{</mo> <msub> <mi>a</mi> <mi>n</mi> </msub> <mo stretchy="false">}</mo> </mrow> </math></EquationSource> </InlineEquation> satisfies a linear recurrence of arbitrary degree with integer coefficients. Under appropriate conditions, we prove that it can be continued to a meromorphic function on the complex <i>s</i>-plane. Thus we may associate a Lerch-type zeta function <InlineEquation ID="IEq3"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="9_2025_2926_Article_IEq3.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="67" /> </InlineMediaObject> <EquationSource Format="TEX">\(\varphi (z,s,x)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>φ</mi> <mo stretchy="false">(</mo> <mi>z</mi> <mo>,</mo> <mi>s</mi> <mo>,</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> to a general recurrence. This subsumes all previous results which dealt only with the ordinary zeta and Hurwitz cases and degrees 2 and 3. Our method generalizes a formula of Ramanujan for the classical Hurwitz–Riemann zeta functions. We determine the poles and residues of <InlineEquation ID="IEq4"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="9_2025_2926_Article_IEq4.gif" Format="GIF" Height="12" Rendition="HTML" Resolution="72" Type="Linedraw" Width="16" /> </InlineMediaObject> <EquationSource Format="TEX">\(\varphi \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>φ</mi> </math></EquationSource> </InlineEquation>, which turn out to be polynomials in <i>x</i>. In addition we study the dependence of <InlineEquation ID="IEq5"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="9_2025_2926_Article_IEq3.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="67" /> </InlineMediaObject> <EquationSource Format="TEX">\(\varphi (z,s,x)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>φ</mi> <mo stretchy="false">(</mo> <mi>z</mi> <mo>,</mo> <mi>s</mi> <mo>,</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> on <i>x</i> and <i>z</i>, and its properties as a function of three complex variables.</p>

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The Lerch-Type Zeta Function of a Recurrence Sequence of Arbitrary Degree

  • Luis Manuel Navas Vicente,
  • Àlvaro Serrano Holgado

摘要

We consider the series \(\sum _{n=1}^{\infty } z^{n} (a_{n} + x)^{-s}\) n = 1 z n ( a n + x ) - s where \(\{a_{n}\}\) { a n } satisfies a linear recurrence of arbitrary degree with integer coefficients. Under appropriate conditions, we prove that it can be continued to a meromorphic function on the complex s-plane. Thus we may associate a Lerch-type zeta function \(\varphi (z,s,x)\) φ ( z , s , x ) to a general recurrence. This subsumes all previous results which dealt only with the ordinary zeta and Hurwitz cases and degrees 2 and 3. Our method generalizes a formula of Ramanujan for the classical Hurwitz–Riemann zeta functions. We determine the poles and residues of \(\varphi \) φ , which turn out to be polynomials in x. In addition we study the dependence of \(\varphi (z,s,x)\) φ ( z , s , x ) on x and z, and its properties as a function of three complex variables.