nLab twisting function





Special and general types

Special notions


Extra structure





The notion of twisting functions is an explicit simplicial formula for a cocycle with values in a simplicial group. The twisted product induced from the twisting function is an explicit simplicial formula for a simplicial principal bundle, a model for a discrete principal ∞-bundle classified by this cocycle.

In a fibre bundle or more generally in a fibration, the fibre ‘twists’ as one goes around a loop in the base space, (the standard simple example is the Möbius band). Such fibre bundles are usually restricted to being ‘locally trivial’, that is locally a product of an open set in the base with the fibre.

In the setting of simplicial homotopy theory, one can attempt to construct analogues of fibre bundles by starting with a base simplicial set X X_\bullet and a fibre F F_\bullet and trying to ‘deform’ the simplicial product X ×F X_\bullet \times F_\bullet to get some non-trivial fibred object. In the Cartan seminars of the period 1956–57 (numdam), (about pages 1–10), a neat solution was described: by leaving all but one of the face maps of the product alone, and deforming the last. The result is a “twisted cartesian product” (see below). The deformation required a simplicial automorphism of the fibre of course, and the resulting twisting function went from the base X X_\bullet to the automorphisms of F F_\bullet, completely mirroring the topological example. The simplicial identities force the twisting function to obey certain equations.


Twisting function

Let X X_\bullet be a simplicial set and G G_\bullet a simplicial group. Then a twisting function ϕ:X G \phi :X_\bullet\to G_\bullet is a family of maps ϕ={ϕ n:X nG n1} n>1\phi=\{\phi_n : X_n\to G_{n-1}\}_{n\gt 1} such that

d 0ϕ(x)=(ϕ(d 0x)) 1ϕ(d 1x) d iϕ(x)=ϕ(d i+1x),i>0, s iϕ(x)=ϕ(s i+1x),i0, ϕ(s 0x)=1 G.\array{ d_0 \phi(x) = (\phi(d_0 x))^{-1} \phi(d_1 x)\\ d_i \phi(x) = \phi(d_{i+1}x), i\gt 0,\\ s_i\phi(x) = \phi(s_{i+1}x), i\geq 0,\\ \phi(s_0 x) = 1_G. }

Twisted Cartesian products

Given a simplicial set F F_\bullet with left G G_\bullet-action, one then defines a twisted Cartesian product, (TCP), X × ϕF X_\bullet \times_\phi F_\bullet with (X × ϕF ) n=X n×F n(X_\bullet \times_\phi F_\bullet)_n = X_n\times F_n and

d i(x,f)=(d ix,d if),i>0 d 0(x,f)=(d 0x,ϕ(x)(d 0f)), s i(x,y)=(s ix,s iy).\array{ d_i(x,f) = (d_i x, d_i f), i\gt 0\\ d_0 (x,f) = (d_0 x, \phi(x)(d_0 f)),\\ s_i(x,y) = (s_i x,s_i y). }

Thus the only difference from the usual Cartesian product of simplicial sets is in d 0d_0.


  • A twisting function ϕ:X G \phi :X_\bullet\to G_\bullet corresponds exactly to a simplicial map from XX to W¯(G )\overline{W}(G_\bullet) delooping of the simplicial group. There is a universal twisting function W¯(G ) G \overline{W}(G_\bullet)_\bullet\to G_\bullet. See simplicial principal bundle for more.

  • By the adjunction between WW-bar and the Dwyer-Kan loop groupoid functor, a twisting function also corresponds to a morphism of simplicial groupoids G(X )G G(X_\bullet)\to G_\bullet.


Twisting functions are the analogue of twisting cochains in the context of simplicial sets; but twisting cochains were introduced by Brown 1959, whilst twisting functions were discussed already in Cartan 1956.


The link between Kan fibrations and simplicial fibre bundles, and thus TCPs is neatly summarised in:


Last revised on November 28, 2023 at 15:20:07. See the history of this page for a list of all contributions to it.