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Note on commutativity in double semigroups and two-fold monoidal categories

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arxiv math/0608452 v1 pith:WPJCFRYB submitted 2006-08-17 math.CT

classification math.CT
keywords doublesemigroupsunitsassociativecategoriescommutativitymonoidalparticular
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A concrete computation -- twelve slidings with sixteen tiles -- reveals that certain commutativity phenomena occur in every double semigroup. This can be seen as a sort of Eckmann-Hilton argument, but it does not use units. The result implies in particular that all cancellative double semigroups and all inverse double semigroups are commutative. Stepping up one dimension, the result is used to prove that all strictly associative two-fold monoidal categories (with weak units) are degenerate symmetric. In particular, strictly associative one-object, one-arrow 3-groupoids (with weak units) cannot realise all simply-connected homotopy 3-types.

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  1. A higher-order Eckmann-Hilton argument

    math.CT 2026-06 unverdicted novelty 7.0 of 10

    Algebraic higher-order Eckmann-Hilton argument deriving braidings from two monoidal structures and symmetries from three, applied to show hom-categories of n-degenerate semi-strict (n+1)-categories are symmetric monoidal.

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