Approximations by special values of multiple cosine and sine functions

Abstract

Kurokawa and Koyama's multiple cosine function $\mathcal{C}_{r}(x)$ and Kurokawa's multiple sine function $S_{r}(x)$ are generalizations of the classical cosine and sine functions from their infinite product representations, respectively. For any fixed $x\in[0,\frac{1}{2})$, let $$B=\left\{\frac{\log\mathcal{C}_{r}(x)}π~~\bigg|~~r=1,2,3,\ldots\right\}$$ and $$C=\left\{\frac{\log S_r(x)}π~~\bigg|~~r=1,2,3,\ldots\right\}$$ be the sets of special values of $\mathcal{C}_{r}(x)$ and $S_{r}(x)$ at $x$, respectively. In this paper, we will show that the real numbers can be strongly approximated by linear combinations of elements in $B$ and $C$ respectively, with rational coefficients. Furthermore, let $$D=\left\{\frac{ζ_{E}(3)}{π^2},\frac{ζ_{E}(5)}{π^4}, \ldots, \frac{ζ_{E}(2k+1)}{π^{2k}},\ldots; \frac{β(4)}{π^3},\frac{β(6)}{π^5}, \ldots, \frac{β(2k+2)}{π^{2k+1}},\ldots\right\}$$ be the set of special values of Dirichlet's eta and beta functions. We will prove that the set $D$ has a similar approximation property, where the coefficients are values of the derivatives of rational polynomials. Our approaches are inspired by recent works of Alkan (Proc. Amer. Math. Soc. 143: 3743--3752, 2015) and Lupu-Wu (J. Math. Anal. Appl. 545: Article ID 129144, 2025) as applications of the trigonometric integrals.