# The carry propagation of the successor function

## TL;DR

This paper investigates the carry propagation phenomenon in various numeration systems, establishing conditions for its existence and calculating its limit, extending classical results from standard base systems to more complex systems.

## Contribution

It introduces a unified approach using combinatorial, algebraic, and ergodic techniques to analyze carry propagation in non-standard numeration systems and provides conditions for its existence.

## Key findings

- The limit of amortized carry propagation exists in several non-standard systems.
- Conditions for the existence of carry propagation are close to necessary.
- Explicit values for carry propagation limits are obtained for certain systems.

## Abstract

Given any numeration system, we call carry propagation at a number $N$ the number of digits that are changed when going from the representation of $N$ to the one of $N+1$, and amortized carry propagation the limit of the mean of the carry propagations at the first $N$ integers, when $N$ tends to infinity, if this limit exists. In the case of the usual base $p$ numeration system, it can be shown that the limit indeed exists and is equal to $p/(p-1)$. We recover a similar value for those numeration systems we consider and for which the limit exists. We address the problem of the existence of the amortized carry propagation in non-standard numeration systems of various kinds: abstract numeration systems, rational base numeration systems, greedy numeration systems and beta-numeration. We tackle the problem by three different types of techniques: combinatorial, algebraic, and ergodic. For each kind of numeration systems that we consider, the relevant method allows for establishing sufficient conditions for the existence of the carry propagation and examples show that these conditions are close to being necessary conditions.