Not signed in

Want to take part in these discussions? Sign in if you have an account, or apply for one below

• Username
• Password
• Remember me

• Sign in using OpenID
• Remember me

• Forgot your password?
• Apply for membership

2-category 2-category-theory abelian-categories adjoint algebra algebraic algebraic-geometry algebraic-topology analysis analytic-geometry arithmetic arithmetic-geometry book bundles calculus categorical categories category category-theory chern-weil-theory cohesion cohesive-homotopy-type-theory cohomology colimits combinatorics complex complex-geometry computable-mathematics computer-science constructive cosmology deformation-theory descent diagrams differential differential-cohomology differential-equations differential-geometry digraphs duality elliptic-cohomology enriched fibration foundation foundations functional-analysis functor gauge-theory gebra geometric-quantization geometry graph graphs gravity grothendieck group group-theory harmonic-analysis higher higher-algebra higher-category-theory higher-differential-geometry higher-geometry higher-lie-theory higher-topos-theory homological homological-algebra homotopy homotopy-theory homotopy-type-theory index-theory integration integration-theory internal-categories k-theory lie-theory limits linear linear-algebra locale localization logic mathematics measure measure-theory modal modal-logic model model-category-theory monad monads monoidal monoidal-category-theory morphism motives motivic-cohomology nlab noncommutative noncommutative-geometry number-theory of operads operator operator-algebra order-theory pages pasting philosophy physics pro-object probability probability-theory quantization quantum quantum-field quantum-field-theory quantum-mechanics quantum-physics quantum-theory question representation representation-theory riemannian-geometry scheme schemes set set-theory sheaf simplicial space spin-geometry stable-homotopy-theory stack string string-theory superalgebra supergeometry svg symplectic-geometry synthetic-differential-geometry terminology theory topology topos topos-theory tqft type type-theory universal variational-calculus

1. • CommentRowNumber1.
   • CommentAuthornLab edit announcer
   • CommentTimeNov 2nd 2022
   • PermaLink
   Author: nLab edit announcer
   Format: MarkdownItexOld discussion in query box: +--{.query} [[Eric]]: Are there any consistency requirements for a 2-morphism? For example, in the bigon above, if $f:a\to b$, $g:a\to b$, and $\alpha:f\to g$, are there requirements on $\alpha:f\to g$ regarding $f$ and $g$? For example, should $\alpha$ come with component 1-morphisms $\alpha_a:a\to a$ and $\alpha_b:b\to b$ such that $$\alpha_a\circ g = f\circ\alpha_b$$ or maybe $$\alpha_a\circ g \simeq f\circ\alpha_b$$ ? Could there be a 2-morphism without the corresponding 1-morphism components? [[Urs Schreiber]]: in any given [[2-category]] you have to specify which 2-morphisms exactly there are supposed to be, what $ \alpha $ exactly you allow between $f$ and $g$. When you ask about components, it seems you are thinking of 2-morphisms specifically in the 2-category [[Cat]]. Here, yes, the allowed 2-morphisms are those that are [[natural transformation]]s between their source and target 1-morphisms, which are [[functor]]s. [[Eric]]: I think the [[exchange law]] might be what I had in mind. Jon Slaughter: Yes, clearly any higher order structure superimposed cannot corrupt the lower structure. Given any $g = \alpha \circ f$ must be a structurally valid/preserving map between $f$ and $g$. As Urs mentions, for Cat the 2-morphisms are natural transforms and must preserve the structure of the functors below them. In general what must be preserved depends on the sub-structure. Since composable morphisms must be reducible, this requires 2-morphisms to preserve the composability rule. For 2-morphisms this does work out to be the exchange laws. For higher order morphisms one can expect compositions of higher order composiblity rules. E.g., if natural transformations must preserve functorality in Cat then 3-morphisms will have to preserve naturality(but in terms of the functor category these 3-morphisms reduce to 2-morphisms and their structural preserving rules reduce to the exchange laws). It generally is not valid to talk about components with n-morphisms since one does not know the object structure. In Cat, with functors, we know by definition they have components(the image of an arrow under the functor) and the natural transformation that must preserve the composibility of components. The exchange laws are more general in that they are not expressed in terms of components. =-- Anonymous <a href="https://ncatlab.org/nlab/revision/diff/2-morphism/21">diff</a>, <a href="https://ncatlab.org/nlab/revision/2-morphism/21">v21</a>, <a href="https://ncatlab.org/nlab/show/2-morphism">current</a>

   Old discussion in query box:

   +–{.query} Eric: Are there any consistency requirements for a 2-morphism? For example, in the bigon above, if $f:a\to b$, $g:a\to b$, and $\alpha:f\to g$, are there requirements on $\alpha:f\to g$ regarding $f$ and $g$? For example, should $\alpha$ come with component 1-morphisms $\alpha_a:a\to a$ and $\alpha_b:b\to b$ such that

   $$\alpha_a\circ g = f\circ\alpha_b$$

   or maybe

   $$\alpha_a\circ g \simeq f\circ\alpha_b$$

   ? Could there be a 2-morphism without the corresponding 1-morphism components?

   Urs Schreiber: in any given 2-category you have to specify which 2-morphisms exactly there are supposed to be, what $\alpha$ exactly you allow between $f$ and $g$. When you ask about components, it seems you are thinking of 2-morphisms specifically in the 2-category Cat. Here, yes, the allowed 2-morphisms are those that are natural transformations between their source and target 1-morphisms, which are functors.

   Eric: I think the exchange law might be what I had in mind.

   Jon Slaughter: Yes, clearly any higher order structure superimposed cannot corrupt the lower structure. Given any $g = \alpha \circ f$ must be a structurally valid/preserving map between $f$ and $g$. As Urs mentions, for Cat the 2-morphisms are natural transforms and must preserve the structure of the functors below them. In general what must be preserved depends on the sub-structure. Since composable morphisms must be reducible, this requires 2-morphisms to preserve the composability rule. For 2-morphisms this does work out to be the exchange laws. For higher order morphisms one can expect compositions of higher order composiblity rules. E.g., if natural transformations must preserve functorality in Cat then 3-morphisms will have to preserve naturality(but in terms of the functor category these 3-morphisms reduce to 2-morphisms and their structural preserving rules reduce to the exchange laws). It generally is not valid to talk about components with n-morphisms since one does not know the object structure. In Cat, with functors, we know by definition they have components(the image of an arrow under the functor) and the natural transformation that must preserve the composibility of components. The exchange laws are more general in that they are not expressed in terms of components.

   =–

   Anonymous

   diff, v21, current