Butterflies I: morphisms of 2-group stacks

Abstract

Weak morphisms of non-abelian complexes of length 2, or crossed modules, are morphisms of the associated 2-group stacks, or gr-stacks. We present a full description of the weak morphisms in terms of diagrams we call butterflies. We give a complete description of the resulting bicategory of crossed modules, which we show is fibered and biequivalent to the 2-stack of 2-group stacks. As a consequence we obtain a complete characterization of the non-abelian derived category of complexes of length 2. Deligne's analogous theorem in the case of Picard stacks and abelian sheaves becomes an immediate corollary. Commutativity laws on 2-group stacks are also analyzed in terms of butterflies, yielding new characterizations of braided, symmetric, and Picard 2-group stacks. Furthermore, the description of a weak morphism in terms of the corresponding butterfly diagram allows us to obtain a long exact sequence in non-abelian cohomology, removing a preexisting fibration condition on the coefficients short exact sequence.

89 pages, comments on weak morphisms moved to new section 4.2, appendix A removed

References in corpus (4)

• Butterflies I: morphisms of 2-group stacks
• On weak maps between 2-groups
• Butterflies II: Torsors for 2-group stacks
• 2-Gerbes bound by complexes of gr-stacks, and cohomology

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