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• CommentRowNumber1.
• CommentAuthorUrs
• CommentTimeNov 1st 2011

added to the Properties-section at Hopf algebra a brief remark on their interpretation as 3-vector spaces.

• CommentRowNumber2.
• CommentAuthorDavid_Corfield
• CommentTimeNov 1st 2011

Are they a particular kind of 3-vector space? I mean is the Hopf algebra construction a categorification of a particular kind of 2-vector space?

And does the 3-dimensionality of a Hopf algebra show itself somehow?

• CommentRowNumber3.
• CommentAuthorzskoda
• CommentTimeNov 2nd 2011
• (edited Nov 2nd 2011)

I do not understand. Page 27 of Lurie et al. is giving an example of a Hopf algebra (group algebra for a finite group) which provides such a structure. I do not see a claim that all Hopf algebras, especially infinite dimensional provide something like that. Namely coalgebras and dual algebras correspond only in finite dimensional situation. Second I do not see why would Hopf having anything to do with this – I mean bialgebra requirement in finite dimensional situation does what can be done, I do not see why would antipode do anything here. Not to mention Hopf algebras in more general tensor categories, where the situation would be even worse.

• CommentRowNumber4.
• CommentAuthorjim_stasheff
• CommentTimeNov 2nd 2011
At the very least, please unpack that 3-vs remark!
but thanks for the Caution about topologist's convention
btw, it should be Heyneman-Sweedler but it's too late now
• CommentRowNumber5.
• CommentAuthorUrs
• CommentTimeNov 2nd 2011

I write out more details when I have a minute. But it’s straightforward to work it out.

• CommentRowNumber6.
• CommentAuthorUrs
• CommentTimeFeb 3rd 2013

coming back to the discussion of Hopf algebras as (bases for) 3-vector spaces:

I see that this is secretly discussed in

• Xiang Tang, Alan Weinstein, Chenchang Zhu, Hopfish algebras, Pacific J. Math. 231 (2007), no. 1, 193–216. (arXiv:math/0510421)

What they call a sesquiunital sesquialgebra in their def. 2.5 is precisely a (basis for a) 3-vector space: an algebra object internal to 2-vector spaces (with basis), hence internal to algebras+bimodules.

Have to run now. Maybe more later.

• CommentRowNumber7.
• CommentAuthorUrs
• CommentTimeFeb 3rd 2013

added brief mentioning and pointer to the literature in Tannaka duality for Hopf algebras

• CommentRowNumber8.
• CommentAuthorUrs
• CommentTimeApr 9th 2013

the entry Hopf algebra could do with some editing.

I started by polishing and expanding the Idea-section a bit. But have to interrupt now.

• CommentRowNumber9.
• CommentAuthorUrs
• CommentTimeMar 15th 2016

These and all related entries (e.g. Hopf algebroid, Steenrod algebra, etc. ) still could do with much editing.

• CommentRowNumber10.
• CommentAuthorzskoda
• CommentTimeMar 16th 2016
• (edited Mar 16th 2016)

I am sorry but I do not see a point of the sentence

In particular a co-group in rings is a Hopf algebra; a fact highlighted by Haynes Miller in the context of discussion of dual Steenrod algebras, see (Ravenel 86, appendix A) for review.

I mean why to mention Haynes Miller, what is so deep about mentioning somebody years after this point of view gave the whole branches of mathematics ? Also the message is not clear. Where is Miller, in Ravanel’s book, and what is the usage of this information ? To say that it was widely used point of view since early 1960s, see for example that M. Kac in early 1960-s started a huge subject of what is now called Kac algebras motivated by this commutative picture and then going beyond into nocommutative. Or Bergmann who had extensive work on cogroups in associative algebra setup, both commutative and noncommutative in 1970s. I. Bernstein had shown in late 1950s, I think, that unlike in commutative algebras, cogroups in unital associative algebras are very rare, that is why people need less categorical regular thing like Hopf algebras, as cogroups in associative algebras are so few. Algebraic geometers of course had thought of the affine group schemes (that is group objects in affine schemes) as spectra of cogroups in commutative algebras by the very definition of the subject from around 1960.

Steenrod algebra is a major historical motivation for Hopf algebras and this has been also used almost at the very beginning of the subject.

Or maybe you just want to say that you learned this from Miller so you want to keep the record in $n$Lab ? (I can not see its universal meaning, so it is probably particular and of less usage for others, unless it has some additional info which is yet not revealed here in the article)

• CommentRowNumber11.
• CommentAuthorzskoda
• CommentTimeMar 16th 2016
• (edited Mar 16th 2016)

The Ravenel’s book asserts that the term Hopf algebroid (for commutative Hopf algebroid) is due Miller. I believe this. But the observation that the cogroups in commutative rings are commutative Hopf rings and cogroups in commutative algebras are commutative Hopf algebras has been widely used way of thinking before Miller and is widely documented (perhaps not in homotopy theory, but in algebra and algebraic geometry).

In any case I will add commutative to all points in the article as it is not true without saying “commutative”: noncommutative Hopf algebras are not cogroups in any category.

• CommentRowNumber12.
• CommentAuthorzskoda
• CommentTimeMar 16th 2016
• (edited Mar 16th 2016)

I changed the critical part in the entry into:

In particular, a co-group in the category of (unital) commutative rings is a commutative Hopf ring and a cogroup in the category of (unital) commutative $k$-algebras is a commutative Hopf $k$-algebra; a fact highlighted in homotopy theory by Haynes Miller (in view of his generalization to commutative Hopf algebroids as cogroupoids in commutative algebra) in the context of discussion of dual Steenrod algebras, see (Ravenel 86, appendix A) for review.

## References

Discussion of commutative Hopf algebras as cogroups is in

• CommentRowNumber13.
• CommentAuthorzskoda
• CommentTimeMar 16th 2016
• (edited Mar 16th 2016)

Here is the Israel Berstein’s (not Joseph Bernstein!) article showing that cogroups in associative algebras are extremely few (unlike Hopf algebras), and are basically free as algebras (the context is also algebraic topology):

• Israel Berstein, On cogroups in the category of graded algebras. Trans. Amer. Math. Soc. 115 (1965), 257–269 jstor
• CommentRowNumber14.
• CommentAuthorzskoda
• CommentTimeMar 16th 2016

This fact is observed in bigger generality by

• Benoit Fresse, Cogroups in algebras over an operad are free algebras, Commen. Math. Helv. 73:4, 1998, 637–676 doi
• CommentRowNumber15.
• CommentAuthorUrs
• CommentTimeMar 16th 2016

Okay, thanks. I thought I had added the “commutative” qualifier where necessary, but thanks for catching places where I missed it.

• CommentRowNumber16.
• CommentAuthorUrs
• CommentTimeJan 14th 2020

Under “Examples” I added the line

the ordinary homology of an H-space (for instance a based loop space) is a Hopf algebra via its Pontrjagin ring-structure

• CommentRowNumber17.
• CommentAuthorUrs
• CommentTimeJan 19th 2021

• W. Stephen Wilson, Hopf rings in algebraic topology, Expositiones Mathematicae, 18:369–388, 2000 (pdf)

(this used to be referenced only at W. S. Wilson and without the pdf link, so I completed the item and copied it to here)

• CommentRowNumber18.
• CommentAuthorUrs
• CommentTimeFeb 21st 2021

• CommentRowNumber19.
• CommentAuthorUrs
• CommentTimeFeb 21st 2021

• CommentRowNumber20.
• CommentAuthorUrs
• CommentTimeMay 10th 2021

I have added mentioning (here) of the notion of involutive Hopf algebra (and will give this its own little page, for ease of hyperlinking).

Then in the proposition (here) that the antipoide is an anti-homomorphism I have added the statement that, hence, involutive Hopf algebras are star-algebras.

• CommentRowNumber21.
• CommentAuthorzskoda
• CommentTimeMay 10th 2021

Following the language of star algebra maybe it is better to say algebra anti-involution as the antipode is an antihomomorphism. I don’t care, just suggestion so maybe resulting in a bit more uniform convention. Set theoretically it is just an involution, but in algebra world there is a distinction.

• CommentRowNumber22.
• CommentAuthorUrs
• CommentTimeMay 11th 2021

Okay, sure, anti-involution.