Targeted Fibonacci Exponentiation

Abstract

A targeted exponentiation algorithm computes a group exponentiation operation $a^k$ with a reversible circuit in such a way that the initial state of the circuit consists of only the base $a$ and fixed values, and the final state consists of only the exponential $a^k$ and fixed values. Three targeted exponentiation algorithms based on Fibonacci addition chains are considered, offering tradeoffs in terms of the number of working registers and the number of iterations. The approaches also motivate related results on the Fibonacci Zeckendorf array, including a new \emph{modular Hofstadter G problem} and an improvement to Anderson's recent algorithm for locating pairs of adjacent integers in the extended Fibonacci Zeckendorf array. The algorithms have applications in quantum computing.