nLab crossed profunctor




Given crossed modules ๐•=(โˆ‚ X:X 1โ†’X 0)\mathbb{X}=(\partial_X : X_1\to X_0), ๐•=(โˆ‚ Y:Y 1โ†’Y 0)\mathbb{Y}=(\partial_Y:Y_1\to Y_0) (actions surpressed from the notation), a crossed profunctor

๐•โ†’๐• \mathbb{X}\to\mathbb{Y}

consists of a group PP and morphisms x 1:X 1โ†’Px_1:X_1\to P, x 0:Pโ†’X 0x_0:P\to X_0, y 1:Y 1โ†’Py_1:Y_1\to P, y 0:Pโ†’Y 0y_0:P\to Y_0

X 1 Y 1 โ†˜ x 1 y 1โ†™ โˆ‚ Xโ†“ P โ†“ โˆ‚ Y โ†™ x 0 y 0โ†˜ X 0 Y 0 \array{ X_1 &&&& Y_1 \\ & \searrow^{\mathrlap{x_1}} & & {}^{\mathllap{y_1}}\swarrow \\ {}^{\mathllap{\partial_X}}\downarrow && P && \downarrow^{\mathrlap{\partial_Y}} \\ & \swarrow_{\mathrlap{x_0}} && {}_{\mathllap{y_0}}\searrow \\ X_0 &&&& Y_0 }

such that

  • the two triangles commute, x 0โˆ˜x 1=โˆ‚ Xx_0\circ x_1=\partial_X, y 0โˆ˜y 1=โˆ‚ Yy_0\circ y_1=\partial_Y,

  • the diagonals compose to identities y 0โˆ˜x 1=1y_0\circ x_1 = 1, x 0โˆ˜y 1=1x_0\circ y_1 = 1,

  • and

    x 1( x 0(p)x)=px 1(x)p โˆ’1,y 1( y 0(p)y)=py 1(y)p โˆ’1, x_1 ({}^{x_0(p)}x) = p x_1(x) p^{-1}, \,\,\,\,\,y_1 ({}^{y_0(p)}y) = p y_1(y)p^{-1} \,,

    where xโˆˆX 1x\in X_1, pโˆˆPp\in P, yโˆˆY 1y\in Y_1.

The complex Y 1โ†’Pโ†’X 0Y_1\to P\to X_0 is called the NE-SW complex, and X 1โ†’Pโ†’Y 0X_1\to P\to Y_0 is called the NW-SE complex.

If to the above definition was added the property that the SE-NW sequence Y 1โ†’Pโ†’X 0Y_1\to P\to X_0 is exact in that it is a (nonabelian in general) group extension, this is called a butterfly. See there for more details.

Alex asks: Is there any relation between a profunctor and a Crossed Profunctor? Just as one could look at a crossed module as a module with โ€œtwisted multiplicationโ€ (according to the crossed module page), is there some analogous โ€œtwistingโ€ of a profunctor to obtain a crossed profunctor?

Beppe says: no. Actually the name โ€œcrossed profunctorโ€ was not a happy choice. A crossed profunctor is just the normalized version of a profunctor, as a crossed module is the normalization of a groupoid. This extends to a strong biequivalence between the bicategory of crossed modules, crossed profunctors and their morphisms (in Grp) and that of crossed modules, profunctors and their morphisms.

This result holds also in intrinsic settings, say when we consider internal categories in a semi-abelian category.


This notion appeared in

  • M. Jibladze, Coefficients for cohomology of โ€œlargeโ€ categories , pp. 169โ€“179, in H. Inassaridze (Ed.), K-theory and Homological Algebra (A Seminar held at the Razmadze Mathematical Institute in Tbilisi, Georgia, USSR 1987-88), Lec. Notes in Math. 1437, Springer 1990 (jibladzeCoeffLargeCats.djvu).

Last revised on January 24, 2018 at 14:42:46. See the history of this page for a list of all contributions to it.