The Weighted Tower of Hanoi: Algebraic Structure, Phase Transitions, and Integer Sequences

TL;DR

This paper presents a comprehensive algebraic framework for the weighted Tower of Hanoi, revealing connections to classical integer sequences, phase transitions in strategies, and explicit solutions for various weight models.

Contribution

It introduces a unified algebraic theory for weighted Hanoi with explicit formulas, spectral analysis, and phase transition phenomena in move strategies.

Findings

Spectral decomposition links one-LDM dynamics to Jacobsthal and Lichtenberg sequences.

Explicit formulas derived for classical sequences like Fibonacci, Lucas, Pell, Euler.

Identifies a phase transition in optimal strategies based on disc size and move costs.

Abstract

We develop a unified algebraic theory of the weighted Tower of Hanoi with arbitrary nonnegative symmetric move costs depending on both disc index and pegs. Starting from a general optimality recurrence with two competing strategies -- one largest-disc move (one-LDM) and two largest-disc moves (two-LDM) -- we derive complete matrix formulations for both regimes and obtain explicit closed forms for the minimal transfer cost. The one-LDM dynamics is governed by a nontrivial linear operator whose spectral decomposition reveals a fundamental connection with the Jacobsthal and Lichtenberg sequences, while the two-LDM dynamics exhibits pure exponential growth. This framework yields exact solutions for broad classes of weight models, including peg-symmetric, disc-symmetric, polynomial, geometric, arithmetic, and sequence-induced costs. In particular, choosing classical integer sequences (Fibonacci, Lucas, Jacobsthal, Pell, Euler, etc.) as disc weights produces new derived sequences with explicit formulas and recurrences, establishing the Tower of Hanoi as a sequence-generating transform. We further introduce and analyze models with forbidden moves and move-type-dependent weights, uncovering a phase transition phenomenon in which the optimal strategy switches from two-LDM behavior for small discs to one-LDM behavior beyond a finite threshold. Our results provide a comprehensive algebraic and combinatorial understanding of weighted Hanoi dynamics and expose deep connections between optimal solutions and classical integer sequences.