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math.QA2010★ 1 cited
Hall Algebras as Hopf Objects
Christopher D. Walker
One problematic feature of Hall algebras is the fact that the standard multiplication and comultiplication maps do not satisfy the bialgebra compatibility condition in the underlyi…
math.QA2008★ 8 cited
Groupoidification Made Easy
John C. Baez, Alexander E. Hoffnung, Christopher D. Walker
Groupoidification is a form of categorification in which vector spaces are replaced by groupoids, and linear operators are replaced by spans of groupoids. We introduce this idea wi…