The Newton approximation, the Hurwitz continued fraction, and the Sierpinski series for relatively quadratic units over certain imaginary quadratic number fields (2510.15498v1)

Abstract: The objective of this paper is to show (a)=(b)=(c) as rational functions of $T$, $U$ for (a), (b), (c) given by (a) continued fractions of length $2^{n+1}-1$ with explicit partial denominators in $\left{-T,U^{-1}T\right}$, (b) truncated series $\sum_{0\le m\le n} \left(U^{2^m}/\left(h_0(T)h_1(T,U) \cdots h_m(T,U)\right)\right)$ with $h_n$ defined by $h_0:=T$ and $h_{n+1}(T,U):=h_n(T,U)^2-2U^{2^n} (n \geq 0)$, (c) $(n+1)$-fold iteration $F^{(n+1)}(0)= F^{(n+1)}(0,T,U)$ of $F(X)= F(X,T,U) :=X-f(X)/\frac{df}{dX}(X)$ for $f(X)=X^2-T X+U$, and to find explicit equalities among truncated Hurwitz continued fraction expansion of relatively quadratic units $\alpha \in \mathbb{C}$ over imaginary quadratic fields $\mathbb{Q}\left(\sqrt{-1}\right)$, $\mathbb{Q}\left(\sqrt{-3}\right)$, rapidly convergent complex series called the Sierpinski series, and the Newton approximation of $\alpha$ on the complex plane. We also give an estimate of the error of the Newton approximation of the unit $\alpha$.