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Of course, not just _any_ third space will do. The topological set up is as follows. We have two [[topological space]]s, $X$ and $Y$, and a [[generalised cohomology theory]] $E^*(-)$. We want to be able to transfer ( _transgress_ ) $E^*$-classes from $X$ to $Y$. We find a third space, $Z$, which has morphisms to both $X$ and $Y$. We usually write this in the following way: $$ \begin{matrix} Z & \to& X \\ \downarrow \\ Y \end{matrix} $$ (Note to self: replace with SVG, write $f \colon Z \to X$ and $g \colon Z \to Y$.) An important example is that where we have some parameter space $\Sigma$ (for instance the circle $\Sigma = S^1$) and set $Z = \Sigma \times [\Sigma,X]$, the [[product]] of $\Sigma$ with the [[internal hom|mapping space]] $[\Sigma,X]$ of maps from $\Sigma$ to $X$ (for instance the [[loop space]] of $X$). Then the morphism on the right is taken to be evaluation and the morphism on the left is projection onto the mapping space $[\Sigma,X]$: $$ \array{ \Sigma \times [\Sigma, X] &\stackrel{ev}{\to}& X \\ \downarrow^{\mathrlap{p_2}} \\ [\Sigma,X] } \,. $$ This setup induces **transgression to mapping spaces** in the following. Now we apply the generalised cohomology theory to this diagram. As it is a cohomology theory, the arrows reverse. We have part of our route from $E^*(X)$ to $E^*(Y)$, namely from $E^*(X)$ to $E^*(Z)$. However, the way from $E^*(Y)$ to $E^*(Z)$ is blocked: it is a one-way street and we want to go the _wrong_ way. However, under certain special circumstances, cohomology theories admit push-forward maps. That is, if $g \colon Z \to Y$ is particularly nice, there is a map $g_! \colon E^*(Z) \to E^*(Y)$. The notation $g_!$ (pronounced "g shriek") has the explanation that it is a surprise that there is a map in this direction[^shriek]. Note that the push-forward map, $g_! \colon E^*(Z) \to E^*(Y)$, often results in a change of degree. This is described at [[fiber integration]] and [[Pontrjagin-Thom collapse map]]. [^shriek]: To the best of [[Andrew Stacey|my]] knowledge, this remark is attributable to [[Ralph Cohen]]. Thus the key to transgression is to understand the conditions where the map $g \colon Z \to Y$ admits a push-forward map. The simplest case is where $Z$ is a product space, $Y \times W$, and $W$ is [[orientable]] for the generalised cohomology theory $E^*(-)$. This means that $W$ has a fundamental class in $E^*(W)$ and evaluation on this class gives a morphism $E^*(Y \times W) \to E^*(Y)$. ## Definition ### Transgression of differential forms See at _[[transgression of differential forms]]_, ### By pullback and fiber integration (...) ### Transgression of forms through fiber bundles A related construction called _transgression_ is the following For $P \to X$ a [[fiber bundle]] with typical fiber $i : F \hookrightarrow P $ and $[\omega] \in H_{dR}^n(F)$ a class in [[de Rham cohomology]] of the fiber, we say this is _transgressive_ if * there exists a form $cs \in \Omega^{n}(P)$ on the total space of the bundle; * such that restricted along $F \hookrightarrow P$ to the fiber it is $\sim \omega$; * and such that $d cs$ is the pull-back of a form $\kappa \in \Omega^{n+1}(X)$ on the base along the bundle projection . See for instance section 9 of ([Borel](#Borel)). This construction really exhibits transgression as a special case of the [[connecting homomorphism]] in cohomology $$ H^n(F) \to H^{n+1}(X) $$ that is induced from the [[short exact sequence]] of cocycles $$ ker(i^*) \hookrightarrow \Omega^\bullet(P) \stackrel{i^*}{\to} \Omega^\bullet(F) $$ by restricting to those elements for which it factors through $$ \Omega^\bullet(X) \hookrightarrow ker(i^*) \,. $$ For more on this see [[Chern-Simons element]]. ## Examples ### Transgression in de Rham Cohomology ### With a [[cohomology theory]] that has a good geometric model, such as [[de Rham cohomology]], there is often a similar geometric model for the push-forward map. In the case of de Rham cohomology, it goes by the name of _[[fiber integration|integration along the fibres]]_. Using the notation above, with the simple case of $Z = Y \times W$, we have the formula $$ \alpha \in H^k_{dR}(X) \mapsto \int_W f^* \alpha \in H^{k - \dim W}_{dR}(Y) $$ ### Transgression of de Rham cohomology to loop spaces A particular application of this is in respect to the cohomology of [[loop spaces]], and in particular to [[iterated integrals]]. The aim here is to transfer information from a space $X$ to its [[free loop space object|free loop space]], $L X$. The intermediate space in this situation is $S^1 \times L X$ with evaluation as the map to $X$ and projection to $L X$. Note that although $L X$ is rather large, the [[fibre]] is $S^1$ and it is that which needs to be controlled. If we worked in the realm of pure [[algebraic topology]] (i.e. didn't care about geometry) this transfer would be almost trivial. Indeed, if we worked with _based_ loops, it would be a tautology since then the push-forward $H^*(S^1 \wedge \Omega X) \to H^{*-1}(\Omega X)$ is simply the suspension isomorphism. (We use the [[smash product]] as we are working with _based_ spaces here.) However, we wish to have a geometric interpretation of this, and so work in the realm of [[differential topology]]: i.e. with smooth [[manifold]]s (albeit possibly infinite dimensional). We therefore have the diagram: $$ \array{ S^1 \times L M & \to & M \\ \downarrow \\ L M } $$ The general lore of transgression says that this induces a map in cohomology $H^k_{dR}(M) \to H^{k-1}_{dR}(L M)$ via $$ \alpha \mapsto \int_{S^1} ev^*\alpha. $$ As with any cohomological construction, there is always the question as to whether or not it is purely cohomological or whether or not it can be lifted to whatever-it-was that defined the cohomology theory. In this case, that is [[differential form]]s. In this case we are in the happy circumstance that the lift exists. Expressed purely in terms of differential forms, the formula doesn't change. However, we can think of differential forms as "that which acts on [[tangent vector]]s" and ask for a formulation that involves tangent vectors. This can make the integration step clearer. To reformulate transgression thus we need to understand tangent vectors on $L M$. The simplest way to view a tangent vector on $L M$ is to identify $T L M$ with $L T M$. That is, we view a tangent vector $x \in T_\gamma L M$ as a loop in $T M$ with the property that $x(t) \in T_{\gamma(t)} M$. In one view ($T L M$), $x$ tells $\gamma$ which direction to move in; in the other view ($L T M$), each $x(t)$ tells each $\gamma(t)$ which direction to move in. That done, we have an obvious evaluation map $S^1 \times T L M \to T M$, $x \mapsto x(t)$ and so, given a differential form on $M$, we can evaluate it on tangent vectors coming from $L M$. However, that's not all we need. There's a very special [[vector field]] on $L M$ which we want to throw into the mix. This is the vector field which assigns to $\gamma \in L M$ the tangent vector $\gamma' \in L T M$. This is the rotation vector field: if we rotate the circle on $S^1 \times L M$ then to keep the evaluation map consistent, we have to rotate the loops in $L M$ as well. So starting with a $k$-form, say $\alpha$, on $M$, we fix a loop $\gamma \in L M$, $t \in S^1$, and take $k-1$ tangent vectors at $\gamma$, say $x_1, \dots, x_{k-1}$. We evaluate these at $t$ to get $x_1(t), \dots, x_{k-1}(t) \in T_{\gamma(t)} M$ and also throw in $\gamma'(t)$. That gives $k$ vectors at $T_{\gamma(t)} M$ on which we can evaluate $\alpha$: $$ \alpha(x_1(t),x_2(t),\dots,x_{k-1}(t),\gamma'(t)) $$ This is a number. Hence, by varying $t \in S^1$, it defines a function $S^1 \to \mathbb{R}$. Integrating this function around $S^1$ yields a number: $$ \int_{S^1} \alpha(x_1(t),x_2(t),\dots,x_{k-1}(t),\gamma'(t)) dt $$ Thus we have a map $\bigotimes^{k-1} T(L M) \to \mathbb{R}$. It can be shown that it is alternating and $(k-1)$-linear, and that it varies smoothly over $L M$, hence defines an element of $\Omega^{k-1}(L M)$. The reason why $\int_{S^1} ev^*\alpha$ takes the form of the above formula is the following: $$ev^*: \Omega^k(M) \to \Omega^k(S^1\times LM)= \oplus_{i+j=k} \Omega^i(S^1) \otimes\Omega^j(LM) = \Omega^0(S^1) \otimes \Omega^k(LM) \oplus \Omega^1(S^1)\otimes \Omega^{k-1}(LM). $$ Thus we may write $$ev^*\alpha= \alpha_0\otimes \alpha_k + \alpha_{1}\otimes \alpha_{k-1}, $$ where $\alpha_k\in \Omega^k(LM)$, $\alpha^{k-1}\in \Omega^{k-1}(LM)$, and $\alpha_1\in \Omega^1(S^1)$, $\alpha^0\in \Omega^0(S^1)$. Then contracting with $S^1$ via $\int_{S^1}$, only the second term survives. Thus \[ \label{int} (\int_{S^1} ev^*\alpha)_{\gamma} (x_1, ..., x_{k-1}) =\int_{S^1} \alpha_{1} \otimes \alpha_{k-1} (x_1, ..., x_{k-1} ) .\] On the other hand, let $(v_i, x_i) \in T_t S^1 \times T_{\gamma}LM $ be tangent vectors of $T(S^1\times LM)$ for $i=1, \dots, k$. Then $$ev^*\alpha_{(t, \gamma)}((v_1, x_1), \dots, (v_k, x_k)) = \alpha_{k-1}|_{\gamma} \otimes \alpha_1 |_t((v_1, x_1), \dots, (v_k, x_k)) + \alpha_k|_{\gamma}\otimes \alpha_0|_t((v_1, x_1), \dots, (v_k, x_k)).$$ At the same time, $T_{\gamma} ev( v, x) = v \gamma' + x$. This can be seen by taking a variation (a small path) $(t+v\epsilon, \gamma^\epsilon)$ representing $(v, x)$ (thus $\gamma^0=\gamma$). Then $$T_{\gamma} ev( v, x)= \frac{d}{d\epsilon}|_{\epsilon=0}(\gamma^\epsilon(t+v\epsilon))=\frac{d}{d\epsilon}|_{\epsilon=0}\gamma^0(t+v\epsilon)+ \frac{d}{d\epsilon}|_{\epsilon=0}\gamma^\epsilon(t) = v\gamma'(t)+x(t),$$ which is a tangent vector at $\gamma(t)$. Thus $$ev^*\alpha|_{(t, \gamma)}((v_1, x_1), \dots, (v_k, x_k))= \alpha_{\gamma(t)}(T_\gamma ev (v_1, x_1), \dots, T_\gamma ev (v_k, x_k)) $$ $$ = \alpha_{\gamma(t)}(x_1(t), \dots, x_k(t)) + \alpha_{\gamma(t)}(v_1 \gamma'(t), x_2(t), \dots, x_{k}(t)) + c.p. $$ Thus $\alpha_k=\alpha$, $\alpha_0=1$, $\alpha_{k-1}=\iota(\gamma')\alpha$, and $\alpha_1=dt$. Combining with equation \eqref{int}, we obtain the desired formula (might be up to a sign). There is much more to tell in this particular story. It is possible to replacing $S^1$ by a [[simplex]] (but keeping $L M$ the same) and so build up more complicated objects in $\Omega^*(L M)$. This is the starting point of Chen's theory of [[iterated integrals]] and further developments of the theory can be found in the work of Jones, Getzler, and Petrack. ### Transgression in group cohomology See at *[[transgression in group cohomology]]*. ### Transgression to mapping spaces in terms of internal homs {#InternalHom} > just a rough remark, for the moment At least for some cases of transgression to [[mapping space]]s, the concept has an [[nPOV]] description in terms of [[internal hom]]s. This makes crucial use of the [[nPOV]] notion of [[cohomology]], as described there. Let $\mathbf{H} = $ [[Top]] $\simeq$ [[∞Grpd]]. For $X \in \mathbf{H}$ and for $\Sigma$ a $k$-dimensional closed oriented [[manifold]], and for $K$ an [[abelian group]] that is injective as a $\mathbb{Z}$-module, we may identify transgression to the mapping space $[\Sigma,X]$ as the composite $$ trans_\Sigma : \mathbf{H}(X,\mathbf{B}^n K) \stackrel{[\Sigma,-]}{\to} \mathbf{H}([\Sigma,X], [\Sigma, \mathbf{B}^n K]) \stackrel{(\tau_{n-k})_*}{\to} \mathbf{H}([\Sigma,X], \tau_{n-k} [\Sigma, \mathbf{B}^n K]) \simeq \mathbf{H}([\Sigma,X], \mathbf{B}^{n-k} K) \,. $$ Here the first step is application of the [[internal hom]], then the second is postcomposition with [[truncated|truncation]], and then the last step uses the [[universal coefficient theorem]]. Note that the transgression map $trans_\Sigma$ depends on the choice of an equivalence $\tau_{n-k} [\Sigma, \mathbf{B}^n K]\simeq \mathbf{B}^{n-k} K$. Up to homotopy, this choice is precisely the datum of the [[orientation]] on $\Sigma$. > more details go here.... This plays a role in the [[quantization]] process that yields [[FQFT]]s. For an application see [[Dijkgraaf-Witten theory]]. > more details, and more polishing later... ## Related entries * [[transgression of bundle gerbes]] ## References The classical notion of transgression of forms through fiber bundles is described in section 9 of * {#Borel} [[Armand Borel]], _Topology of Lie groups and characteristic classes_ Bull. Amer. Math. Soc. Volume 61, Number 5 (1955), 397-432. ([EUCLID](http://projecteuclid.org/euclid.bams/1183520007)) Transgression as [[pullback in cohomology]] along the [[evaluation map]] followed by [[fiber integration]] over the base space is considered in: for [[differential forms]]: * [[Jean-Luc Brylinski]], §3.5 in *Loop Spaces, Characteristic Classes, and Geometric Quantization*, Birkhäuser (1993) &lbrack;[doi:10.1007/978-0-8176-4731-5](https://doi.org/10.1007/978-0-8176-4731-5)&rbrack; for [[Deligne cohomology]] (and with an eye towards defining the [[higher holonomy]] of [[bundle gerbes with connection]]): * [[Ernesto Lupercio]], [[Bernardo Uribe]], p. 2 of: *Holonomy for Gerbes over Orbifolds*, J. Geom.Phys. **56** (2006) 1534-1560 &lbrack;[arXiv:math/0307114](https://arxiv.org/abs/math/0307114), [doi:10.1016/j.geomphys.2005.08.006](https://doi.org/10.1016/j.geomphys.2005.08.006)&rbrack; > (over [[orbifolds]]) * [[Christian Bär]], [[Christian Becker]], Def. 9.1 in: *Differential Characters and Geometric Chains*, in: *Differential Characters*, Lecture Notes in Mathematics **2112**, Springer (2014) &lbrack;[arXiv:1303.6457](https://arxiv.org/abs/1303.6457), [doi:10.1007/978-3-319-07034-6_1](https://doi.org/10.1007/978-3-319-07034-6_1)&rbrack; Transgression of [[group cohomology]] for [[discrete groups]] to cohomology of [[inertia groupoids]] (see at *[[transgression in group cohomology]]*): * {#Willerton08} [[Simon Willerton]], Section 1 of: *The twisted Drinfeld double of a finite group via gerbes and finite groupoids*, Algebr. Geom. Topol. 8 (2008) 1419-1457 ([arXiv:math/0503266](https://arxiv.org/abs/math/0503266)) * [[Jean-Louis Tu]], [[Ping Xu]], Section 3 of: *The ring structure for equivariant twisted K-theory*, J. Reine Angew. Math. 635 (2009), 97–148 ([arXiv:math/0604160](https://arxiv.org/abs/math/0604160), [doi:10.1515/CRELLE.2009.077](https://doi.org/10.1515/CRELLE.2009.077)) * {#AdemRuanZhang07} [[Alejandro Adem]], [[Yongbin Ruan]], [[Bin Zhang]], Section 4 of: _A Stringy Product on Twisted Orbifold K-theory_, Morfismos (10th Anniversary Issue), **11** 2 (2007) 33-64 &lbrack;[arXiv:math/0605534](https://arxiv.org/abs/math/0605534), [Morfismos pdf](www.morfismos.cinvestav.mx/Portals/morfismos/SiteDocs/Articulos/Volumen11/No2/Zhang/arz.pdf)&rbrack; The [[image]] of the transgression operation from [[bundle gerbes]] ([[bundle gerbe with connection|with connection]]) to [[complex line bundles]] ([[connection on a vector bundle|with connection]]) on the [[free loop space]] of their base space is characterized (as consisting of the "[[fusion bundle|fusion bundles]]") in: * [[Konrad Waldorf]], *Transgression to Loop Spaces and its Inverse, I: Diffeological Bundles and Fusion Maps*, Cah. Topol. Geom. Differ. Categ., 2012, Vol. LIII, 162-210 &lbrack;[arXiv:0911.3212](https://arxiv.org/abs/0911.3212), [cahierstgdc:LIII](http://cahierstgdc.com/index.php/volumes/volume-liii-2012)&rbrack; * [[Konrad Waldorf]], *Transgression to Loop Spaces and its Inverse, II: Gerbes and Fusion Bundles with Connection*, Asian Journal of Mathematics **20** 1 (2016) 59-116 &lbrack;[arXiv:1004.0031](https://arxiv.org/abs/1004.0031), [doi:10.4310/AJM.2016.v20.n1.a4](https://dx.doi.org/10.4310/AJM.2016.v20.n1.a4)&rbrack; * [[Konrad Waldorf]], *Transgression to Loop Spaces and its Inverse, III: Gerbes and Thin Fusion Bundles*, Advances in Mathematics **231** (2012) 3445-3472 &lbrack;[arXiv:1109.0480](https://arxiv.org/abs/1109.0480), [doi:10.1016/j.aim.2012.08.016](https://doi.org/10.1016/j.aim.2012.08.016)&rbrack; [[!redirects transgressions]] </textarea> <p> <input id="alter_title" name="alter_title" onchange="toggleVisibility();" type="checkbox" value="1" /> <label for="alter_title">Change page name.</label><br/> <span id="title_change" style="display:none"><label for="new_name">New name:</label> <input id="new_name" name="new_name" onblur="addRedirect();" type="text" value="transgression" /></span> </p> <div> <p style="font-size: 0.8em; width: 70%;"> For non-trivial edits, please briefly describe your changes below. 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