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Transactions of the American Mathematical Society

ISSN 1088-6850(online) ISSN 0002-9947(print)

 
 

 

Strong forms of self-duality for Hopf monoids in species


Author: Eric Marberg
Journal: Trans. Amer. Math. Soc. 368 (2016), 5433-5473
MSC (2010): Primary 18D10, 18D35; Secondary 16T30
DOI: https://doi.org/10.1090/tran/6506
Published electronically: December 2, 2015
MathSciNet review: 3458387
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Abstract: A vector species is a functor from the category of finite sets with bijections to vector spaces (over a fixed field); informally, one can view this as a sequence of $ S_n$-modules, one for each natural number $ n$. A Hopf monoid (in the category of vector species) consists of a vector species with unit, counit, product, and coproduct morphisms satisfying several compatibility conditions, analogous to a graded Hopf algebra.

A vector species has a basis if and only if it is given by a sequence of $ S_n$-modules which are permutation representations. We say that a Hopf monoid is freely self-dual if it is connected and finite-dimensional, and if it has a basis in which the structure constants of its product and coproduct coincide. Such Hopf monoids are self-dual in the usual sense, and we show that they are furthermore both commutative and cocommutative.

We prove more specific classification theorems for freely self-dual Hopf monoids whose products (respectively, coproducts) are linearized in the sense that they preserve the basis; we call such Hopf monoids strongly self-dual (respectively, linearly self-dual). In particular, we show that every strongly self-dual Hopf monoid has a basis isomorphic to some species of block-labeled set partitions, on which the product acts as the disjoint union. In turn, every linearly self-dual Hopf monoid has a basis isomorphic to the species of maps to a fixed set, on which the coproduct acts as restriction. It follows that every linearly self-dual Hopf monoid is strongly self-dual.

Our final results concern connected Hopf monoids which are finite-
dimensional, commutative, and cocommutative. We prove that such a Hopf monoid has a basis in which its product and coproduct are both linearized if and only if it is strongly self-dual with respect to a basis equipped with a certain partial order, generalizing the refinement partial order on set partitions.


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Additional Information

Eric Marberg
Affiliation: Department of Mathematics, Stanford University, Stanford, California 94305
Email: emarberg@stanford.edu

DOI: https://doi.org/10.1090/tran/6506
Received by editor(s): January 14, 2014
Received by editor(s) in revised form: June 30, 2014
Published electronically: December 2, 2015
Article copyright: © Copyright 2015 American Mathematical Society

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