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The \newvocab{identity functor} #{\id_{\C}: \C \to \C} (denoted also by #{ 1_{\C}: \C \to \C}), defined by #{\id_{\C}(X)=X} and #{\id_{\C}(f)=f} for every #{X \in \Ob(\C)} and every #{f \in \Arr(\C)}.
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}
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\p{
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The \newvocab{constant functor} #{\Delta_O: \C \to \D} which assigns a fixed #{O \in \mathrm{Ob}(\D)} to any object of #{\C} and #{\id_O}, the identity arrow on #{O}, to any arrows from #{\C} :
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\tikz{
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\begin{tikzcd}
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X &&&& O \\
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& {} && {} \\
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Y &&&& O \\
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\C &&&& \D
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\arrow["f"', from=1-1, to=3-1]
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\arrow["{\id_O}", from=1-5, to=3-5]
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\arrow["{\Delta_O}", Rightarrow, maps to, from=2-2, to=2-4]
\taxon{example}\refnotet{other special functors}{3.1, example 10, 11, 4.6}{kostecki2011introduction}{
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\p{Some other special functors are introduced in later sections in context, e.g. \vocabk{hom-functor}{tt-001S}, \vocabk{Yoneda embedding functors}{tt-002T}, \vocabk{object functor}{tt-003H}.
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with compositions and identities preserved in a trivial way.}
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\p{The \newvocab{forgetful functor}, which \em{forgets} some part of structure, however arrows, compositions and identities are preserved.
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\p{
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Let #{\C} be a subcategory of #{\D}. The \newvocab{inclusion functor}, denoted #{\hookrightarrow}, sends objects and arrows of #{\D} into themselves in category #{\D}.}
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\p{
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The \newvocab{diagonal functor} #{\Delta: \C \to \C \times \C, \Delta(X)=(X, X)} and #{\Delta(f)=(f, f)} for #{f: X \to X^{\prime}}.
Copy file name to clipboardExpand all lines: trees/tt-0027.tree
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Let #{\C^{\J}} be a [functor category](tt-001F), where #{\J} is a [small category](tt-000A).
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\p{Let #{\Delta_O} be a [constant functor](tt-0016), which assigns the same object #{O} in #{\C} to any object #{J} in #{\J}.
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\p{Let #{\Delta_O} be a [constant functor](tt-003Q), which assigns the same object #{O} in #{\C} to any object #{J} in #{\J}.
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}
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\p{Let #{K \in \Ob(\J)} and let #{j \in \operatorname{Arr}(\J)} such that #{j: J \to K}. Let #{\fF} be any functor in #{\C^{\J}}, i.e. it's a [diagram](tt-0025) in #{\C} of shape #{\J}.
Copy file name to clipboardExpand all lines: trees/tt-0028.tree
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commutes.
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\p{The family of arrows are components of a \vocabk{natural transformation}{tt-001E} #{\pi: \Delta_V \to \fD}, i.e. from the \vocabk{constant functor}{tt-0016} ( which assigns the same object #{V} to any object #{J_i} in #{\J}) to diagram functor #{\fD}.
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\p{The family of arrows are components of a \vocabk{natural transformation}{tt-001E} #{\pi: \Delta_V \to \fD}, i.e. from the \vocabk{constant functor}{tt-003Q} ( which assigns the same object #{V} to any object #{J_i} in #{\J}) to diagram functor #{\fD}.
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\p{For simplicity, we refer to a cone by "a cone #{(V, \pi)} on #{\fD}".
% definition theorem lemma construction observation
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% convention corollary axiom example exercise proof
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% discussion remark notation
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\refdeft{identity functor}{3.1, example 1}{kostecki2011introduction}{
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\p{
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The \newvocab{identity functor} #{\id_{\C}: \C \to \C} (denoted also by #{ 1_{\C}: \C \to \C}), defined by #{\id_{\C}(X)=X} and #{\id_{\C}(f)=f} for every #{X \in \Ob(\C)} and every #{f \in \Arr(\C)}.
% definition theorem lemma construction observation
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% convention corollary axiom example exercise proof
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% discussion remark notation
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\refdeft{constant functor}{3.1, example 2}{kostecki2011introduction}{
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\p{
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The \newvocab{constant functor} #{\Delta_O: \C \to \D} which assigns a fixed #{O \in \mathrm{Ob}(\D)} to any object of #{\C} and #{\id_O}, the identity arrow on #{O}, to any arrows from #{\C} :
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\tikz{
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\begin{tikzcd}
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X &&&& O \\
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& {} && {} \\
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Y &&&& O \\
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\C &&&& \D
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\arrow["f"', from=1-1, to=3-1]
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\arrow["{\id_O}", from=1-5, to=3-5]
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\arrow["{\Delta_O}", Rightarrow, maps to, from=2-2, to=2-4]
% definition theorem lemma construction observation
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% convention corollary axiom example exercise proof
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% discussion remark notation
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\refdeft{inclusion functor}{3.1, example 4}{kostecki2011introduction}{
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\p{
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Let #{\C} be a \vocabk{subcategory}{tt-002Y} of #{\D}. The \newvocab{inclusion functor}, denoted #{\hookrightarrow}, sends objects and arrows of #{\D} into themselves in category #{\D}.}
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