typical contexts
In a local topos there is a notion of concrete objects. These form a reflective subcategory. The corresponding reflector is the concretification map which universally approximates any object by a concrete object.
There is a unique evident notion of concretification in any local topos, this we discuss first in
This involves an image factorization. Since in higher category theory/homotopy theory image factorization refines to a tower of notions of n-image factorization in a local (∞,1)-topos there are different constructions that one may all think of as concretification. This we discuss in
A local topos is a topos equipped with a sharp modality $\sharp$.
For $X$ any object of the topos, the image projection of the unit $\iota_X \colon X \to \sharp X$ is the concretification of $X$
In a local (∞,1)-topos there is still the sharp modality, but here the 1-image-factorization of its unit is rarely of interest, for this concretifies an object in degree 0 but makes it codiscrete in all higher degrees. Typically one is interested in concretifying in all degrees. One needs to specify extra data to say what this means.
One case where this is arises is the differential concretification of moduli ∞-stacks of principal ∞-connections.
The following gives a sufficient condition for modeling n-image factorizations in some (∞,1)-toposes with particularly convenient presentation.
Let $C$ be a site with enough points, so that the weak equivalences in $sPSh(C)_{\mathrm{loc}}$ are detected on stalks (this prop.). Then given a morphism of Kan complex-valued simplicial presheaves
such that both $X$ and $Y$ are homotopy k-types for some finite $k \in \mathbb{N}$, then its n-image factorization in the (∞,1)-topos $L_{lwhe} sPSh(C)_{loc}$ for any $n \in \mathbb{N}$ is presented by any factorization $X \longrightarrow im_{n}(f) \longrightarrow Y$ in $sPSh(C)$ through some Kan-complex valued simplicial presheaf $im_n(f)$ such that for each object $U \in C$ the simplicial homotopy groups satisfy the following conditions:
$\pi_{\bullet \lt n}\left(X(U) \to (im_{n}(f))(U)\right)$ are isomorphisms;
$\pi_n\left(X(U) \to (im_{n}(f))(U)\to Y(U)\right)$ is the (epi,mono) factorization of $\pi_n(f(U))$;
$\pi_{\bullet \gt n}\left((im_{n}(f))(U) \to Y(U)\right)$ are isomorphisms.
Evalutation on stalks is a filtered colimit which preserves the finite limits and finite colimits that go into the definition of simplicial homotopy groups. Therefore the global conditions assumed on the simplicial homotopy groups imply that the same kind of conditions holds for the stalkwise homotopy groups. These are the categorical homotopy groups in $L_{lwhe} sPSh(C)_{loc}$. By this prop. and this def. we may recognize $n$-truncation of morphisms on categorical homotopy groups (using the assumption that $X$ and $Y$ are $k$-truncated for some $k$). Therefore the claim now follows from the stalkwise long exact sequence of homotopy groups.
In order to appeal to prop. we are interested in explicit models for $n$-image factorization of morphisms of Kan complexes. The following gives such for the special case that the the morphism of Kan complexes is the image under the Dold-Kan correspondence of a chain map between chain complexes.
Let $f_\bullet \colon V_\bullet \longrightarrow W_\bullet$ be a chain map between chain complexes
For $n \in \mathbb{N}$, consider the abelian group
For the following it is helpful to think of this abelian group in the following equivalent ways.
Define an equivalence relation on $V_n$ by
Then
is equivalently the set of equivalence classes of this equivalence relation, which inherits abelian group structure since the eqivalence relation is linear.
This is because the equivalence relation says equivalently that
and hence is generated under linearity by
Moreover, notice that the Dold-Kan correspondence
factors through globular strict omega-groupoids (here). An n-morphism in the strict omega-groupoid $DK(V_\bullet)$ is of the form
In terms of these morphisms the equivalence relation above says that two of them are equivalent precisely if
they are “parallel morphisms” in that they have the same source and target;
they have the same image under $f$ in the n-morphisms of $DK(W_\bullet)$.
This suggests yet another equivalent way to think of $(im_{n+1}(f))_n$: it is the disjoint union over the target $(n-1)$-cells in $V_{n-1}$ of the images under $f$ of the sets of $n$-cells from zero to that target:
Let $f_\bullet \colon V_\bullet \longrightarrow W_\bullet$ be a chain map between chain complexes and let $n \in \mathbb{N}$. Recall the abelian group $\underset{v_{n-1}}{\sqcup}\{f_n(v_n) \vert \partial v_n = v_{n-1}\}$ from remark .
The following diagram of abelian groups commutes:
Moreover, the middle vertical sequence is a chain complex $im_{n+1}(f)_\bullet$, and hence the diagram gives a factorization of $f_\bullet$ into two chain maps
Finally, this is a model for the (n+1)-image factorization of $f$ in that on homology groups the following holds:
$H_{\bullet \lt n}(V) \overset{\simeq}{\to} H_{\bullet \lt n}(im_{n+1}(f))$ are isomorphisms;
$H_n(V) \to H_n(im_{n+1}(f)) \hookrightarrow H_n(W)$ is the image factorization of $H_n(f)$;
$H_{\bullet \gt n}(im_{n+1}(f)) \overset{\simeq}{\to} H_{\bullet \gt n}(W)$ are isomorphisms.
This follows by elementary and straightforward direct inspection.
For $p \in \mathbb{N}$ and $k \leq p+1$ write
for the simplicial presheaf which is the image under the Dold-Kan correspondence of the presheaf of chain complexes which is the Deligne complex starting with the presheaf represented by $U(1)$ in degree $p+1$ and truncated to the differential $k$-forms, as shown.
Since the $DK$ map sends surjections of chain complexes to Kan fibrations, the canonical projection maps yield a tower of objectwise Kan fibrations of the following form:
For $\Sigma$ a smooth manifold, write
for the image under the Dold-Kan correspondence of the presheaf of chain complexes which to $U \in CartSp$ assigns the vertical Cech-Deligne complex on $\Sigma \times U \to U$ in the given degree, i.e. the Cech-Deligne complex involving differential forms on $\Sigma \times U$ that have no leg along $U$, i.e. those in $\Omega^{\bullet,0}(\Sigma \times U)$.
We first consider differential concretification on geometrically contractible base spaces. Once this is established, then the general differential concretification follows simply by stackifying along the base space.
(differential concretification for higher circle connections on contractibles)
Let $\Sigma$ be a contractible smooth manifold. For $p \in \mathbb{N}$ write
and then for $0 \leq k \leq p$ define inductively
Let $\Sigma$ be a contractible smooth manifold. Then there is a weak equivalence
from the inductively defined object from def. to the moduli object from def. .
By the assumption that $\Sigma$ is contractible, the Cech-direction of the Cech-Deligne double complex is trivial and so we have for all $U \in CartSp$ and $0 \leq k \leq p$ weak equivalences of the form
and
We claim now for all $k \leq p$ that
For $k = p$ this is the statement to be shown. Hence we may now prove this by induction.
It is manifestly true for $k = 0$. Hence suppose it is true for some $k \lt p$. Observe then that
is an objectwise Kan fibration, because so is $\mathbf{B}^{p+1}U(1)_{conn^{k+1}} \to \mathbf{B}^{p+1}U(1)_{conn^k}$ by def. , and both $[\Sigma,-]$ as well as $\sharp$ are right Quillen functors from $sPSh(C)$ with its global projective model structre to itself.
It follows (this prop.) that the homotopy fiber product in question is presented by the plain 1-categorical fiber product. Since $DK$ is right adjoint, this in turn is given by the degreewise fiber product of the corresponding chain complexes. By direct inspection this means that
Hence we are now reduced to computing the $(p+1-k)$ image of
Observe that in degree $(p+1)-(k+1)$ the ordinary image is
(differential concretification of moduli for higher connection)
For $\Sigma$ a smooth manifold, define for $p \in \mathbb{N}$
to be the homotopy limit over the differential concretifications from def. of contractibles $U_i$, for
a presentation of $\Sigma$ as a homotopy colimit of contractible manifolds (e.g. the realization of the Cech nerve of a good open cover).
For $\Sigma$ a smooth manifold, then the differential concretifiction of def. is equivalent to the moduli object from def. :
Let $\Sigma \simeq \underset{\longrightarrow}{\lim}_i^h U_i$ be the realization of the Cech nerve of a good open cover. Then
is equivalently the image under DK of the corresponding Cech hypercomplex with coefficients in the presheaf of chain complexes $(\mathbf{B}^p U(1))\mathbf{Conn}_{p+1}(-)$. By lemma this is the vertical Deligne complex, and hence the claim follows.
Introductory lecture notes with an eye towards applications in fundamental physics are at
The differential concretification of differential moduli is discussed in
Last revised on June 25, 2020 at 13:22:04. See the history of this page for a list of all contributions to it.