Sums of infinite series involving the Dirichlet lambda function
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arXiv
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| Format: | Preprint |
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2025
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| _version_ | 1866911052195692544 |
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| author | Hu, Su Kim, Min-Soo |
| author_facet | Hu, Su Kim, Min-Soo |
| contents | The Dirichlet lambda function $λ(s)$ is defined for $\mathrm{Re}(s) > 1$ by \[ λ(s) = \sum_{n=0}^{\infty} \frac{1}{(2n+1)^s}. \] This function was initially studied by Euler on the real line, where he denoted it by $N(s)$.
In this paper, by applying the partial fraction decomposition of $π\tan(πx)$ and explicit evaluations of the integrals \[ \int_0^{\frac{1}{2}} x^{2m-1} \cos(2lπx) dx \quad \text{and} \quad \int_0^{\frac{1}{2}} x^{m-1} \log \cos(πx) dx, \] for positive integers $l$ and $m$, we derive closed-form expressions for several classes of infinite series involving $λ(s)$. We also demonstrate that the values $λ(k)$ for even integers $k \geq 2$ arise as constant terms in the Fourier expansions of Eisenstein series associated with the congruence subgroup \[ Γ_0(2) := \left\{ \begin{pmatrix}
a & b
c & d \end{pmatrix} \in \operatorname{SL}_2(\mathbb{Z}) : c \equiv 0 \pmod{2} \right\}. \] |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2504_08347 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Sums of infinite series involving the Dirichlet lambda function Hu, Su Kim, Min-Soo Number Theory Mathematical Physics Classical Analysis and ODEs 11M06 The Dirichlet lambda function $λ(s)$ is defined for $\mathrm{Re}(s) > 1$ by \[ λ(s) = \sum_{n=0}^{\infty} \frac{1}{(2n+1)^s}. \] This function was initially studied by Euler on the real line, where he denoted it by $N(s)$. In this paper, by applying the partial fraction decomposition of $π\tan(πx)$ and explicit evaluations of the integrals \[ \int_0^{\frac{1}{2}} x^{2m-1} \cos(2lπx) dx \quad \text{and} \quad \int_0^{\frac{1}{2}} x^{m-1} \log \cos(πx) dx, \] for positive integers $l$ and $m$, we derive closed-form expressions for several classes of infinite series involving $λ(s)$. We also demonstrate that the values $λ(k)$ for even integers $k \geq 2$ arise as constant terms in the Fourier expansions of Eisenstein series associated with the congruence subgroup \[ Γ_0(2) := \left\{ \begin{pmatrix} a & b c & d \end{pmatrix} \in \operatorname{SL}_2(\mathbb{Z}) : c \equiv 0 \pmod{2} \right\}. \] |
| title | Sums of infinite series involving the Dirichlet lambda function |
| topic | Number Theory Mathematical Physics Classical Analysis and ODEs 11M06 |
| url | https://arxiv.org/abs/2504.08347 |