Arxiv:2008.00486V2 [Math.CT] 1 Nov 2020
Anticommutativity and the triangular lemma. Michael Hoefnagel Abstract For a variety V, it has been recently shown that binary products com- mute with arbitrary coequalizers locally, i.e., in every fibre of the fibration of points π : Pt(C) → C, if and only if Gumm’s shifting lemma holds on pullbacks in V. In this paper, we establish a similar result connecting the so-called triangular lemma in universal algebra with a certain cat- egorical anticommutativity condition. In particular, we show that this anticommutativity and its local version are Mal’tsev conditions, the local version being equivalent to the triangular lemma on pullbacks. As a corol- lary, every locally anticommutative variety V has directly decomposable congruence classes in the sense of Duda, and the converse holds if V is idempotent. 1 Introduction Recall that a category is said to be pointed if it admits a zero object 0, i.e., an object which is both initial and terminal. For a variety V, being pointed is equivalent to the requirement that the theory of V admit a unique constant. Between any two objects X and Y in a pointed category, there exists a unique morphism 0X,Y from X to Y which factors through the zero object. The pres- ence of these zero morphisms allows for a natural notion of kernel or cokernel of a morphism f : X → Y , namely, as an equalizer or coequalizer of f and 0X,Y , respectively. Every kernel/cokernel is a monomorphism/epimorphism, and a monomorphism/epimorphism is called normal if it is a kernel/cokernel of some morphism.
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