Extension of tetration to real and complex heights

TL;DR

This paper introduces a continuous tetrational function for real heights and bases, providing explicit formulas and analyzing its extension to complex domains, revealing limitations on simultaneous complex extension of height and base.

Contribution

It explicitly formulates the continuous tetrational function for real heights and bases, and clarifies the domain restrictions for complex extensions.

Findings

The function is continuous in the real plane.

Complex extension of the base is limited to integer heights.

Complex extension of the height is limited to non-integer real parts.

Abstract

The continuous tetrational function ${^x}r=\tau(r,x)$, the unique solution of equation $\tau(r,x)=r^{\tau(r,x-1)}$ and its differential equation $\tau'(r,x) =q \tau(r,x) \tau'(r,x-1)$, is given explicitly as ${^x}r=\exp_{r}^{\lfloor x \rfloor+1}[\{x\}]_q$, where $x$ is a real variable called height, $r$ is a real constant called base, $\{x\}=x-\lfloor x \rfloor$ is the sawtooth function, $\lfloor x \rfloor$ is the floor function of $x$, and $[\{x\}]_q=(q^{\{x\}}-1)/(q-1)$ is a q-analog of $\{x\}$ with $q=\ln r$, respectively. Though ${^x}r$ is continuous at every point in the real $r-x$ plane, extensions to complex heights and bases have limited domains. The base $r$ can be extended to the complex plane if and only if $x\in \mathbb{Z}$. On the other hand, the height $x$ can be extended to the complex plane at $\Re(x)\notin \mathbb{Z}$. Therefore $r$ and $x$ in ${^x}r$ cannot be complex values simultaneously. Tetrational laws are derived based on the explicit formula of ${^x}r$.