nLab
final lift

Final and initial lifts

Definition

Let U:CDU\colon C\to D be a functor.

Definition

A UU-structured sink is a sink of the form {f i:U(X i)Y}\{f_i \colon U(X_i) \to Y\} in DD.

Note that like all sinks, a UU-structured sink is not necessarily assumed to be small.

Given such a structured sink, let {f i}/D\{f_i\}/D denote the category whose objects are objects ZCZ\in C equipped with a morphism h:YU(Z)h\colon Y\to U(Z) in DD and a sink {g i:X iZ}\{g_i\colon X_i \to Z\} in CC, such that U(g i)=hf iU(g_i) = h \circ f_i for all ii. (Its morphisms are, of course, morphisms in CC commuting with the structure.)

Note that if UU is faithful, then it suffices to demand merely that the g ig_i exist, rather than giving them as part of the structure.

Definition

A semi-final lift of {f i:U(X i)Y}\{f_i \colon U(X_i) \to Y\} is an initial object of {f i}/D\{f_i\}/D. If hh is an isomorphism, then it is called a final lift. If hh is an identity, we call it a strictly final lift.

If objects of CC are regarded as objects of DD equipped with structure, in a (strictly) final lift we say that ZZ is the final structure or strong structure on YY induced by the sink. Note that if UU is an isofibration, then any final lift may be made into a strictly final one.

The dual concept, which applies to cosinks (“sources”), is called a (perhaps semi- or strictly) initial lift, an initial structure or a weak structure.

Examples

  • If DD is the terminal category, then final (or semi-final) lifts reduce to simply colimits in CC.

  • An empty UU-structured sink is just an object of DD, and a semi-final lift of such a sink is a free object on DD. Thus UU admits semi-final lifts of empty sinks precisely when it has a left adjoint. Similarly, it admits final lifts of empty sinks precisely when it has a fully faithful left adjoint (i.e. it admits discrete objects).

  • A singleton UU-structured sink is just a morphism of the form f:U(X)Yf\colon U(X) \to Y. A strictly final lift of such a sink is precisely an opcartesian arrow lying over ff. Thus UU admits strictly final lifts of singleton structured sinks precisely when it is a Grothendieck opfibration (and final lifts of such sinks precisely when it is a Street opfibration).

  • A topological concrete category is a functor that admits final lifts of all (not necessarily small) structured sinks. This turns out to be equivalent to admitting initial lifts of all structured cosinks. The most famous example is then initial topologies and final topologies for U:TopSetU\colon Top \to Set.

  • More generally, a solid functor is one that admits semi-final lifts of all structured sinks.

  • If UU has both a left and right adjoint, of which one (and hence also the other) is fully faithful, and CC is cocomplete, then UU admits final lifts of all small structured sinks. See adjoint triple for a proof. Dually, if CC is complete in this situation, then UU admits initial lifts of all small structured cosinks.

Remarks

Revised on December 15, 2011 21:21:09 by Mike Shulman (71.136.253.8)