nLab (infinity,1)-pushout

Idea

An $(\infty,1)$-pushout is a colimit in an (∞,1)-category $\mathcal{C}$ over a diagram of the shape

$$\{a \leftarrow c \to b\} \to \mathcal{C} \,.$$

In other words it is a cocone

$$\array{ C &\to& B \\ \downarrow &\cong\swArrow& \downarrow \\ A &\to& A \sqcup_C B }$$

which is universal among all such cocones in the $(\infty,1)$-categorical sense.

This is the analog in (∞,1)-category theory of the notion of pushout in category theory.

Incarnations

In model categories

• homotopy pushout

In simplicial type theory

For Segal types

In simplicial type theory, the Segal types are the types that represent (infinity,1)-precategories. Let $A$ be a Segal type in simplicial type theory, and let $x:A$, $y:A$, $z:A$ be two elements in $A$ and let $f:\mathrm{hom}_A(x, y)$ and $g:\mathrm{hom}_A(x, z)$ be two morphisms. The $(\infty,1)$-pushout of $f$ and $g$ is an tuple consisting of

• an element $x \sqcup^z_{f, g} y:A$,

• a morphism $\mathrm{inl}(f, g):\mathrm{hom}_A(y, x \sqcup^z_{f, g} y)$,

• a morphism $\mathrm{inr}(f, g):\mathrm{hom}_A(z, x \sqcup^z_{f, g} y)$,

• and a morphism $\mathrm{compin}(f, g):\mathrm{hom}_A(x, x \sqcup^z_{f, g} y)$

such that

$$\mathrm{inl}(f, g) \circ f = \mathrm{compin}(f, g) \qquad \mathrm{and} \qquad \mathrm{inr}(f, g) \circ g = \mathrm{compin}(f, g)$$

and for any other element $w:A$ with morphisms $h:\mathrm{hom}_A(y, w)$, $j:\mathrm{hom}_A(z, w)$, and $k:\mathrm{hom}_A(x, w)$ such that $h \circ f = k$ and $j \circ g = k$, there exists a unique morphism $l(z, h, j, k):\mathrm{hom}_A(x \sqcup^z_{f, g} y, z)$ such that

$$h = l(z, h, j, k) \circ \mathrm{inl}(f, g) \qquad \mathrm{and} \qquad j = l(z, h, j, k) \circ \mathrm{inr}(f, g) \qquad \mathrm{and} \qquad k = l(z, h, j, k) \circ \mathrm{compin}(f, g)$$

For arbitrary types

The notion of an $(\infty,1)$-pushout can be generalised from Segal types to arbitrary types in simplicial type theory, which represent simplicial infinity-groupoids. In a Segal type $A$, given morphisms $f:\mathrm{hom}_A(x, y)$, $g:\mathrm{hom}_A(y, z)$, and $h:\mathrm{hom}_A(x, z)$, $g \circ f = h$ if and only if $h$ is the unique composite of $f$ and $g$. This means that we can use the latter condition in the definition of an $(\infty,1)$-pushout in types which are not Segal and thus do not have a composition function on morphisms.

Thus, let $A$ be a type in simplicial type theory, and let $x:A$, $y:A$, $z:A$ be two elements in $A$ and let $f:\mathrm{hom}_A(x, y)$ and $g:\mathrm{hom}_A(x, z)$ be two morphisms. The $(\infty,1)$-pushout of $f$ and $g$ is an tuple consisting of

• an element $x \sqcup^z_{f, g} y:A$,

• a morphism $\mathrm{inl}(f, g):\mathrm{hom}_A(y, x \sqcup^z_{f, g} y)$,

• a morphism $\mathrm{inr}(f, g):\mathrm{hom}_A(z, x \sqcup^z_{f, g} y)$,

• and a morphism $\mathrm{compin}(f, g):\mathrm{hom}_A(x, x \sqcup^z_{f, g} y)$

such that

• $\mathrm{compin}(f, g)$ is the unique composite of $\mathrm{inl}(f, g)$ and $f$ and the unique composite of $\mathrm{inr}(f, g)$ and $g$,

• and for any other element $w:A$ with morphisms $h:\mathrm{hom}_A(y, w)$, $j:\mathrm{hom}_A(z, w)$, and $k:\mathrm{hom}_A(x, w)$ such that $k$ is the unique composite of $h$ and $f$ and the unique composite of $j$ and $g$, there exists a unique morphism $l(z, h, j, k):\mathrm{hom}_A(x \sqcup^z_{f, g} y, z)$ such that $h$ is the unique composite of $l(z, h, j, k)$ and $\mathrm{inl}(f, g)$, $j$ is the unique composite of $l(z, h, j, k)$ and $\mathrm{inr}(f, g)$, and $k$ is the unique composite of $l(z, h, j, k)$ and $\mathrm{compin}(f, g)$.

The fact that this definition works for any type $A$ implies that $(\infty,1)$-pushouts should be definable in any simplicial infinity-groupoid or simplicial object in an (infinity,1)-category, not just the $(\infty,1)$-categories or category objects in an (infinity,1)-category.

Related concepts

• pushout

• (infinity,1)-pullback

• (infinity,1)-coproduct

• (infinity,1)-coequalizer

• (infinity,1)-colimit

References

$(\infty,1)$-pushouts are defined in:

• Jacob Lurie, section 4.4.2 of: Higher Topos Theory, Annals of Mathematics Studies 170, Princeton University Press 2009 (pup:8957, pdf)

• Denis-Charles Cisinski, Bastiaan Cnossen, Hoang Kim Nguyen, Tashi Walde, section 5.6 of: Synthetic Category Theory, work in progress (pdf)