nLab
idempotent complete (infinity,1)-category

Context

Idempotents

(,1)(\infty,1)-Category theory

Contents

Idea

An ordinary category is idempotent complete, aka Karoubi complete or Cauchy complete , if every idempotent splits. Since the splitting of an idempotent is a limit or colimit of that idempotent, any category with all finite limits or all finite colimits is idempotent complete.

In an (∞,1)-category the idea is the same, except that the notion of idempotent is more complicated. Instead of just requiring that ee=ee\circ e = e, we need an equivalence eeee\circ e \simeq e, together with higher coherence data saying that, for instance, the two derived equivalences eeeee\circ e\circ e \simeq e are equivalent, and so on up. In particular, being idempotent is no longer a property of a morphism, but structure on it.

It is still true that a splitting of an idempotent in an (,1)(\infty,1)-category is a limit or colimit of that idempotent (now regarded as a diagram with all its higher coherence data), but this limit is no longer a finite limit; thus an (,1)(\infty,1)-category can have all finite limits without being idempotent-complete.

Definition

Definition

Let for a linearly ordered set JJ the simplicial set Δ J:=N(J)\Delta^J:=N(J) be defined to be the nerve of JJ; i.e. Δ J\Delta^J is given in degree nn given by nondecreasing maps {0,...,n}J\{0,...,n\}\to J.

The simplicial set Idem +Idem^+ is defined as follows: for every nonempty, finite, linearly ordered set J, sSet(Δ J,Idem +)sSet(\Delta^J,Idem^+) is the set of pairs (J 0,)(J_0,\sim), where J 0JJ_0\subseteq J and \sim is an equivalence relation on J 0J_0 which satisfies the following condition:

  • Let ijki \le j \le k be elements of JJ such that i,kJ 0i, k \in J_0 , and iki \sim k. Then jJ 0j \in J_0 , and ijki \sim j \sim k.

Let IdemIdem denote the simplicial subset of Idem +Idem^+, corresponding to those pairs (J 0,)(J_0 , \sim) such that J=J 0J = J_0 . Let RetIdem +Ret \subseteq Idem^+ denote the simplicial subset corresponding to those pairs (J 0,)(J_0 , \sim) such that the quotient J 0/J_0 / \sim has at most one element.

(Lurie, 4.4.5.2 p.249)

Definition

Let C be an \infty-category, incarnated as a quasi-category.

  1. An idempotent morphism in C is a map of simplicial sets IdemCIdem \to C. We will refer to Fun(Idem,C)Fun(Idem, C) as the (,1)(\infty,1)-category of idempotents in CC.

  2. A weak retraction diagram in CC is a homomorphism of simplicial sets RetCRet \to C. We refer to Fun(Ret,C)Fun(Ret, C) as the (,1)(\infty,1)-category of weak retraction diagrams in CC.

  3. A strong retraction diagram in CC is a map of simplicial sets Idem +CIdem^+ \to C. We will refer to Fun(Idem+,C)Fun(Idem+, C) as the (,1)(\infty,1)-category of strong retraction diagrams in CC.

(Lurie, 4.4.5.4 p.250)

Definition

An idempotent F:IdemCF \colon Idem \to C is effective if it extends to a map Idem +CIdem^+ \to C.

(Lurie, above corollary 4.4.5.14)

Proposition

An idempotent diagram F:IdemCF \colon Idem \to C is effective precisely if it admits an (∞,1)-limit, equivalently if it admits an (∞,1)-colimit.

By (Lurie, lemma 4.3.2.13).

Definition

CC is called an idempotent complete (,1)(\infty,1) if every idempotent is effective.

(Lurie, above corollary 4.4.5.14)

Properties

The following properties generalize those of idempotent-complete 1-categories.

Theorem

A small (∞,1)-category is idempotent-complete if and only if it is accessible.

This is HTT, 5.4.3.6.

Theorem

For CC a small (∞,1)-category and κ\kappa a regular cardinal, the (∞,1)-Yoneda embedding CCInd κ(C)C \to C' \hookrightarrow Ind_\kappa(C) with CC' the full subcategory on κ\kappa-compact objects exhibits CC' as the idempotent completion of CC.

This is HTT, lemma 5.4.2.4.

References

Section 4.4.5 of

Revised on November 16, 2012 00:42:53 by Guillaume Brunerie (192.16.204.210)