Improved lower bounds of the time complexity of shellsort

Abstract

In this paper we develop the framework of using a parametrized mapping $[σ(1), σ(2), \cdots, σ(n)] \mapsto σ(1)z + σ(2)z^2 + \cdots σ(n)z^n$ to perform runtime analysis on Shellsort. In particular, we show that the worst-case time complexity of Shellsort using Tokuda's gap sequence proposed in 1992 is at least $Ω(N^{1.26})$ with a generalisation of this result to any strictly decreasing gap sequence where each term at most a fixed distance away from a rational geometric sequence, and we also show that strictly decreasing gap sequences giving worst-case Shellsort time complexities of $O(N \log^c N)$ must have $Ω(\log N / \log \log N)$ terms of order $Ω(N / (\log \log N)^c)$.