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Transcendence and continued fraction expansion of values of Hecke-Mahler series. (English) Zbl 1536.11108

Let \(\theta\) and \(\rho\) be real numbers with \(0\leq \theta, \rho <1\) and \(\theta\) irrational. For a complex number \(z\) with \(|z|<1\), set \[ h_{\theta,\rho}(z)=\sum_{k\geq 1} \lfloor k\theta+\rho \rfloor z^k. \] This function is a generalization of the Hecke function \(h_{\theta}(z)\). Similarly, the function \[ F_{\theta,\rho}(z_1,z_2) =\sum_{k_1 \geq 1} \sum_{k_2=1}^{\lfloor k_1\theta+\rho \rfloor} z_1^{k_1} z_2^{k_2} \] generalizes the Mahler function \(F_{\theta}(z_1,z_2)\). The following theorem is proved in the article.
Let \(\theta\) and \(\rho\) be real numbers with \(0 \leq \theta, \rho \leq 1\) and \(\theta\) irrational. Let \(\alpha,\beta\) be nonzero complex algebraic numbers such that \(|\beta \alpha^\theta|<1\) and \(\beta \neq 1\). Then the complex numbers \(\xi_{\mathbf{s}_{\theta,\rho}}(\beta,\alpha)\) and \(\xi_{\mathbf{s'}_{\theta,\rho}}(\beta,\alpha)\) are transcendental. In particular, if \(|\beta|<1\), then the complex numbers \[ h_{\theta,\rho}(\beta), \; F_{\theta,\rho}(\beta,\alpha) \] are transcendental, where \[ \xi_{\mathbf{s}_{\theta,\rho}}(\beta,\alpha)=\sum_{n \geq 1} \big(\lceil n\theta+\rho\rceil -\lceil (n-1)\theta +\rho\rceil \big) \beta^n \alpha^{\lfloor n\theta+\rho\rfloor}, \]
\[ \xi_{\mathbf{s'}_{\theta,\rho}}(\beta,\alpha)=\sum_{n \geq 1} \big(\lfloor n\theta+\rho\rfloor -\lfloor (n-1)\theta +\rho\rfloor \big) \beta^n \alpha^{\lfloor n\theta+\rho\rfloor} \] Let \(a\) and \(b\) be positive integers with \(b \geq 2\). The continued fraction expansion of the real number of the following form \[ (b-1)\xi_{\mathbf{s}_{\theta,\rho}}(1/b,1/a) \quad \text{or} \quad (b-1) \xi_{\mathbf{s'}_{\theta,\rho}}(1/b,1/a), \] is obtained in the paper.

MSC:

11J04 Homogeneous approximation to one number
11J70 Continued fractions and generalizations
11J81 Transcendence (general theory)

References:

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[16] Strasbourg, France and Institut Universitaire de France E-mail: bugeaud@math.unistra.fr
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