nLab algebraic pattern




An algebraic pattern is a blueprint for a notion of functors on a fixed category satisfying a Segal condition, suitable for formalizing homotopy-coherent algebra in the Cartesian setting.



An algebraic pattern is an (โˆž,1)-category ๐’ช\mathcal{O} together with the following data:

  • a pair of wide subcategories ๐’ช int,๐’ช actโŠ‚๐’ช\mathcal{O}^{\mathrm{int}},\mathcal{O}^{\mathrm{act}} \subset \mathcal{O}, whose morphisms are called inert and active morphisms, and

  • a full subcategory ๐’ช elโŠ‚๐’ช int\mathcal{O}^{\mathrm{el}} \subset \mathcal{O}^{\mathrm{int}}, whose objects are called elementary objects,

subject to the condition that for all f:Xโ†’Xโ€ฒf:X \rightarrow X' in ๐’ช\mathcal{O}, the space of factorizations of ff of the form Xโ†’iYโ†’aXโ€ฒX \xrightarrow{i} Y \xrightarrow{a} X' with iโˆˆ๐’ช inti \in \mathcal{O}^{\mathrm{int}} and rโˆˆ๐’ช actr \in \mathcal{O}^{\mathrm{act}} is contractible.

The pair (๐’ช int,๐’ช act)(\mathcal{O}^{\mathrm{int}},\mathcal{O}^{\mathrm{act}}) is a factorization system, with no assumptions made about orthogonality.

It is customary to abusively refer to the quadruple (๐’ช,๐’ช int,๐’ช act,๐’ช el)(\mathcal{O},\mathcal{O}^{\mathrm{int}},\mathcal{O}^{\mathrm{act}}, \mathcal{O}^{\mathrm{el}}) simply as ๐’ช\mathcal{O}, and to decorate inert morphisms as โ†ฆ\mapsto and active morphisms as โ‡\rightsquigarrow. These are so defined in order to define Segal objects.


Let ๐’ช\mathcal{O} be an algebraic pattern and ๐’ž\mathcal{C} a complete (โˆž,1)-category. A Segal ๐’ช\mathcal{O}-object in ๐’ž\mathcal{C} is a functor

F:๐’ชโ†’๐’ž F:\mathcal{O} \rightarrow \mathcal{C}

subject to the condition that, for all Zโˆˆ๐’ชZ \in \mathcal{O}, the canonical morphism

F(Z)โ†’lim Eโˆˆ๐’ช elEโ†ฆZF(E) F(Z) \rightarrow \lim_{\stackrel{E \mapsto Z}{E \in \mathcal{O}^{\mathrm{el}}}}F(E)

is an equivalence.

By Lemma 2.9 of Chu-Hangseng, these are equivalently given as functors F:๐’ชโ†’๐’žF:\mathcal{O} \rightarrow \mathcal{C} whose restriction to ๐’ช int\mathcal{O}^{\mathrm{int}} are right Kan extended from ๐’ช el\mathcal{O}^{\mathrm{el}}.

One example of this is the pattern ๐”ฝ * โ™ญ\mathbb{F}_*^{\flat}, whose underlying category is the category ๐”ฝ *\mathbb{F}_* of finite pointed sets with

  • inert morphisms given by those ฯ•:โŸจnโŸฉโ†’โŸจmโŸฉ\phi:\langle n \rangle \rightarrow \langle m \rangle with |ฯ† โˆ’1(j)|=1|\varphi^{-1}(j)| = 1 for all jโ‰ *j \neq *,

  • active morphisms given by those ฯ•\phi with ฯ• โˆ’1(*)={*}\phi^{-1}(*) = \{*\}, and

  • elementary objects spanned by โŸจ1โŸฉ\langle 1 \rangle.

The resulting (โˆž,1)-category of Segal ๐”ฝ * โ™ญ\mathbb{F}_*^{\flat}-objects in ๐’ž\mathcal{C} consists of the ฮ“-objects in ๐’ž\mathcal{C}; hence they are equivalent to Eโˆž algebras in ๐’ž\mathcal{C} with the cartesian symmetric monoidal structure.

In particular, applying this to the category Cat โˆž\mathrm{Cat}_\infty of small (โˆž,1)-categories and applying unstraightening yields a distinguished subcategory of cocartesian fibrations over ๐’ช\mathcal{O} satisfying Segal conditions. These are sometimes called Segal fibrations.

Relaxing the condition that Segal fibrations have underlying cocartesian fibrations then yields the following notion, originally called weak Segal fibrations.


Let ๐’ช\mathcal{O} be an algebraic pattern. A functor ฯ€:๐’ซโ†’๐’ช\pi:\mathcal{P} \rightarrow \mathcal{O} is called a fibrous pattern if it satisfies the following conditions:

  • ๐’ซ\mathcal{P} has ฯ€\pi-cocartesian lifts for inert morphisms in ๐’ช\mathcal{O}.

  • For all Zโˆˆ๐’ชZ \in \mathcal{O}, the canonical map

    ๐’ซร— ๐’ช๐’ช /Z actโ†’๐’ช /Z actร— lim Eโ†ฆZ๐’ช /E act(lim Eโ†ฆZ๐’ซร— ๐’ช๐’ช /E act,)\mathcal{P} \times_{\mathcal{O}} \mathcal{O}^{\mathrm{act}}_{/Z} \rightarrow \mathcal{O}^{\mathrm{act}}_{/Z} \times_{ \lim_{E \mapsto Z} \mathcal{O}^{\mathrm{act}}_{/E}} \left(\lim_{E \mapsto Z} \mathcal{P} \times_{\mathcal{O}} \mathcal{O}^{\mathrm{act}}_{/E},\right)

    is an equivalence, where the limits run across all inert maps Eโ†ฆZE \mapsto Z with elementary domain.

One of the main reasons to use algebraic patterns over the categorical patterns of Higher Algbera is the relative ease of constructing functoriality in families of examples. The central concept driving this is that of a Segal morphism.


A functor of algebraic patterns F:๐’ชโ†’๐’ซF:\mathcal{O} \rightarrow \mathcal{P} is called a

  • Segal morphism if the pullback functor F *:Fun(๐’ซ,๐’ž)โ†’Fun(๐’ช,๐’ž)F^*:\mathrm{Fun}(\mathcal{P}, \mathcal{C}) \rightarrow \mathrm{Fun}(\mathcal{O}, \mathcal{C}) preserves Segal objects for all ๐’ž\mathcal{C}, and a

  • strong Segal morphism if for all Zโˆˆ๐’ชZ \in \mathcal{O}, the associated functor ๐’ช /Z elโ†’๐’ช /F(Z) el\mathcal{O}^{\mathrm{el}}_{/Z} \rightarrow \mathcal{O}^{\mathrm{el}}_{/F(Z)} is initial.

By Lemma 4.5 of Chu-Haugseng, for a functor of algebraic patterns to be a Segal morphism, it suffices to check that F *F^* preserves Segal spaces. As developed in Barkan-Haugseng-Steinebrunner, (strong) Segal morphisms induce functors on fibrous patterns in the presence of mild categorical โ€œsoundnessโ€ or โ€œextendabilityโ€ conditions on the patterns involved.


  • By Proposition 4.1.7 of Barkan-Haugseng-Steinebrunner, ๐”ฝ * โ™ญ\mathbb{F}_*^{\flat}-fibrous patterns are precisely the (โˆž,1)-operads; defining the pattern structure ๐”ฝ * โ™ฎ\mathbb{F}_*^{\natural} on ๐”ฝ *\mathbb{F}_* with the usual inert and active morphisms together with โˆ…,โŸจ1โŸฉ\emptyset, \langle 1 \rangle as elementary objects, the ๐”ฝ * โ™ฎ\mathbb{F}_*^{\natural}-fibrous patterns are the generalized (โˆž,1)-operads.

  • More generally, when G is a finite group, the G-category of finite pointed G-sets admits a similar pattern structure ๐”ฝฬฒ G,* โ™ญ\underline{\mathbb{F}}_{G,*}^{\flat}, whose Segal objects are the G-commutative monoids and whose fibrous patterns are precisely the G-โˆž-operads.

Both of these examples are more easily presented via span patterns. The theorem relating these is Proposition 5.2.14 of Barkan-Haugseng-Steinebrunner. They define a pattern Span(๐”ฝ G)\mathrm{Span}(\mathbb{F}_G) whose inert maps are backwards, active maps are forwards, and elementary objects are transitive G-sets.


The forgetful functor ๐”ฝ G,*โ†’Span(๐”ฝ G)\mathbb{F}_{G,*} \rightarrow \mathrm{Span}(\mathbb{F}_G) lifts to a Segal morphism of patterns, which induces an equivalence on the categories of Segal obejcts and fibrous patterns.

  • The category ฮ” op\Delta^{\op} has two pattern structures ฮ” op,โ™ฎ,ฮ” op,โ™ญ\Delta^{\op,\natural}, \Delta^{\op, \flat}, which parameterize Segal spaces (i.e. โˆž-categories) and E1 algebras, respectively.

  • The nn-fold product ฮ” op,n,โ™ฎ\Delta^{\op, n, \natural} parameterizes nn-fold Segal spaces, i.e. n-fold โˆž-categories; the nn-fold product ฮ” op,n,โ™ญ\Delta^{\op, n, \flat} models ๐”ผ n\mathbb{E}_n-spaces.

  • The nn-fold wreath product ฮ˜ n:=ฮ”โ‰€โ‹ฏโ‰€ฮ”\Theta_n := \Delta \wr \cdots \wr \Delta has on its opposite two interesting pattern structures; the pattern ฮ˜ n op,โ™ฎ\Theta_n^{\op,\natural} models (โˆž,n)-categories, and the pattern ฮ˜ n op,โ™ญ\Theta_n^{\op,\flat} models ๐”ผ n\mathbb{E}_n-spaces.

  • Let ๐’ฎ m\mathcal{S}_m denote the โˆž\infty-category of m-finite spaces. Then, the pattern Span(๐’ฎ m)\mathrm{Span}(\mathcal{S}_{m}) models m-commutative monoids.

  • The opposite ฮฉ op\Omega^{\op} of the dendroidal category of Moerdijk-Weiss has a pattern structure ฮฉ op,โ™ฎ\Omega^{\op,\natural} whose segal objects are the Dendroidal Segal spaces of Cisinski-Moerdijk, hence they are operads.


Original references:

Slice patterns and morita equivalences appear in:

Last revised on April 19, 2024 at 18:14:40. See the history of this page for a list of all contributions to it.