typical contexts
(2,1)-quasitopos?
structures in a cohesive (∞,1)-topos
Inside a local (∞,1)-topos $\mathbf{H}$ there are objects that may be thought of as ∞-groupoids equipped with extra structure (“cohesive structure” if $\mathbf{H}$ is even a cohesive (∞,1)-topos). These are the concrete objects in $\mathbf{H}$.
Let $\mathbf{H}$ be a local (∞,1)-topos.
Since $Codisc$ is by definition a full and faithful (∞,1)-functor this means that
is a geometric embedding. By the discussion at reflective sub-(∞,1)-category this means that $\Gamma$ is the localization of an (∞,1)-category at a class $S \in Mor \mathbf{H}$ of morphisms. It factors therefore canonically through the (∞,1)-quasitopos of $S$-separated $(\infty,1)$-sheaves
We call $Conc(\mathbf{H})$ the (∞,1)-quasitopos of concrete objects of the local $(\infty,1)$-topos $\mathbf{H}$.
We say an object $X$ is $n$-concrete if the canonical morphism $X \to coDisc \Gamma X$ is (n-1)-truncated.
If a 0-truncated object $X$ is $0$-concrete, we call it just concrete.
For $C$ an ∞-cohesive site, a 0-truncated object in the (∞,1)-topos over $C$ is concrete precisely if it is a concrete sheaf in the traditional sense.
For $X \in \mathbf{H}$ and $n \in \mathbb{N}$, the $(n+1)$-concretification of $X$ is the morphism
that is the left factor in the decomposition with respect to the n-connected/n-truncated factorization system of the $(\Gamma \dashv coDisc)$-unit
By that very n-connected/n-truncated factorization system we have that $conc_{n+1} X$ is an ${n+1}$-concrete object.
concrete $(\infty,1)$-sheaf
This entry goes back to some observations by David Carchedi.
Last revised on November 23, 2011 at 17:50:00. See the history of this page for a list of all contributions to it.