# nLab separable functor

## Motivation

The original motivation was the functorial Maschke's theorem over rings and its various cousins: namely the classical Maschke’s theorem about finite group rings over fields, generalizes to the statements that when the order of the group is invertible in the ground ring then the splitting of an exact sequence of $kG$-module can be obtained functorially from its splitting as an exact sequence of $k$-modules. Functors similar to the forgetful functor ${}_{kG} Mod\to {}_{k}Mod$ in the sense of having such a functorial Maschke’s property are abstracted into the notion of a separable functor.

Similar phenomena appear in the study of ring extensions $f: R\to S$: such a ring extension is separable iff restriction of scalars $\operatorname{Res}_R^S: {}_{S}Mod \to {}_{R}Mod$ is a separable functor.

## Definition

Let $F:C\to D$ be a functor. There is a corresponding (bi)natural transformation with components

$\mathcal{F}_{x,y} = C(x,y)\to D(Fx,Fy),\,\,\,\,(x\stackrel{f}\to y)\mapsto (Fx\stackrel{Ff}\to Fy).$

We way that $F$ is a separable functor if $\mathcal{F}$ splits (i.e. $\mathcal{F}_{x,y}$ has a section natural in $x$ and $y$).

## Properties

Rafael’s theorem. Let $F\dashv G$ be a pair of adjoint functors. Then $F$ is separable iff the unit $\eta:1\to GF$ has a section (= a natural transformation $\nu$ which is its right inverse, $\nu\circ\eta = 1$). $G$ is separable iff the counit $\epsilon:FG\to 1$ has a retraction (= a natural transformation $\zeta$ which is its left inverse, $\eta\circ\zeta =1$).

## Literature

Separable functors were defined in

• C. Năstăsescu, M. van den Bergh, F. van Oystaeyen, Separable functors applied to graded rings, J. Algebra 123 (1989), 397-413, [doi].

Now there is a monograph available:

• S. Caenepeel, G. Militaru, S. Zhu, Frobenius and separable functors for generalized module categories and nonlinear equations, Lec. Notes in Math. 1787, Springer 2002. xiv+354 pp.

Other references

• M. D. Rafael, Separable functors revisited, Comm. in Algebra 18 (1990), 1445-1459.

• S. Caenepeel, B. Ion, G. Militaru, The structure of Frobenius algebras and separable algebras, $K$-Theory 19 (2000), no. 4, 365–402.

• J. Gómez-Torrecillas, Separable functors in corings, Int. J. of Math. and Math. Sci. 30 (2002), 4, Pages 203-225, doi, pdf

• A. Ardizzoni, Separable functors and formal smoothness, J. K-Theory 1 (2008), no. 3, 535–582, math.QA/0407095, doi, MR2009k:16069

• A. Ardizzoni, G. Böhm, C. Menini, A Schneider type theorem for Hopf algebroids, J. Algebra 318 (2007), no. 1, 225–269 MR2008j:16103 doi math.QA/0612633 (arXiv version is unified, corrected); Corrigendum, J. Algebra 321:6 (2009) 1786-1796 MR2010b:16060 doi

The following article studies formal smoothness and generalizes the separable functors to the context of the so-called S-categories which are introduced therein:

• T. Brzeziński, Notes on formal smoothness, in: Modules and Comodules (series Trends in Mathematics). T Brzeziński, JL Gomez Pardo, I Shestakov, PF Smith (eds), Birkhäuser, Basel, 2008, pp. 113-124 (doi, arXiv:0710.5527)

Last revised on July 27, 2017 at 19:09:52. See the history of this page for a list of all contributions to it.