nLab duoid

Contents

Idea

A double monoid [Aguiar & Mahajan (2010) or duoid [Batanin & Markl (2012)] is an object of a duoidal category equipped with two compatible monoid structures.

Definitions

With a binary operation and an element

Naively, a duoid is a set DD equipped with a pair of monoid structures (,I)(\otimes, I) and (,J)(\odot, J) such that:

(vx)(yz)=(vy)(xz). (v \otimes x) \odot (y \otimes z) \;=\; (v \odot y) \otimes (x \odot z) \,.

In this form, a duoid can be viewed as a strict double category with a single object, one horizontal morphism, and one vertical morphism.

However, due to the Eckmann–Hilton argument, this is equivalent to a commutative monoid. In fact, this is true for a duoid in any braided monoidal category (viewed as a duoidal category in which both tensor products coincide).

In a duoidal category

See duoidal category.

References

Duoids were introduced as “double monoids” in the context of duoidal categories in :

  • Marcelo Aguiar, Swapneel Arvind Mahajan, Definition 6.28 of: Monoidal functors, species and Hopf algebras 29 Providence, RI: American Mathematical Society (2010) [pdf, ams:crmm-29]

The name “duoid”, following a suggestion by Ross Street, is due to:

.

Last revised on July 12, 2023 at 07:59:08. See the history of this page for a list of all contributions to it.