# The coalgebra extension problem for $\mathbb Z/p$

## TL;DR

This paper investigates the extension problem for coalgebras over fields, especially circulant coalgebras related to [rac], revealing how the solutions depend on the field's characteristic and exploring connections with algebraic groups and deformations.

## Contribution

It provides the first detailed analysis of the coalgebra extension problem for circulant coalgebras over arbitrary fields, including characteristic-dependent results and explicit conjectures.

## Key findings

- Extension solutions depend on the characteristic of the base field.
- Connections established between coalgebra extensions and algebraic groups like S^1.
- Constructs a formal group as a second-order deformation of polynomial algebra.

## Abstract

For a coalgebra $C_k$ over field $k$, we define the "coalgebra extension problem" as the question: what multiplication laws can we define on $C_k$ to make it a bialgebra over $k$? This paper answers this existence-uniqueness question for certain coalgebras called "circulant coalgebras". We begin with the trigonometric coalgebra, comparing and contrasting with the group-(bi)algebra $k[\mathbb Z/2]$. This leads to a generalization, the dual coalgebra to the group-algebra $k[\mathbb Z/p]$, which we then investigate. We show connections with other questions, motivating us to answer to the coalgebra extension problem for these families. The answer depends interestingly on the base field $k$'s characteristic. Along similar lines, we investigate the algebraic group $S^1$ over arbitrary $k$. We find that similar complications arise in characteristic 2. We explore this, motivated (by quantum groups) by the question of whether or not $\mathcal{O}(S^1)$ is pointed. We give a very explicit conjecture in terms of the Chebyshev polynomials of trigonometry. We end by constructing a formal group object, in a certain monoidal category of modules of $k[[h]]$, as a $2^{\text{nd}}$ order deformation of $k[t]$.