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The generators $d_i, s_i$ for morphisms of the simplicial category satisfy simplicial identities:

$d_jd_i = d_id_{j−1}$ for $i < j$

$s_jd_i = d_is_{j−1}$ for $i < j$

$s_jd_i = id$ for $i = j$, $i = j + 1$

$s_jd_i = d_{i−1}s_j$ for $i > j + 1$

$s_js_i = s_is_{j+1}$ for $i ≤ j$.

Has anyone seen something written about a 2-category with the same objects, the same generators for morphisms, and the simplicial identities replaced with invertible 2-cell which generate the second dimensional structure and satisfy some "coherence conditions"?

I also wonder weather there is any useful 2-category which is obtained by replacing the simplicial identities with non-invertible 2-cells which satisfy some sort of "coherence". In this case one should pay attention to the choice of direction of each of these 2-cell.

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    $\begingroup$ One reasonable way of defining coherent simplicial objects in a (2, 1)-category $\mathfrak{K}$ is to define it to be a morphism from the nerve of $\mathbf{\Delta}^\mathrm{op}$ to the homotopy-coherent nerve of $\mathfrak{K}$ (regarded as a simplicially enriched category). More generally one could do this for any simplicially enriched category, or even a quasicategory, which is how Lurie defines groupoid objects in §6.1.2 of Higher topos theory. $\endgroup$
    – Zhen Lin
    Apr 24 '14 at 21:13
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Todd's answer to your second question is excellent. As for your first question, you may want to check out Steve Lack's paper A Coherent Approach to Pseudomonads. He constructs a 2-category $\Delta'$ which has the same relationship to pseudomonads that the 1-category $\Delta$ has to ordinary monads and the 2-category $\Delta$ has to lax-idempotent 2-monads. His definition is not in terms of generators and relations but rather of an "all diagrams commute" sort, but he then proves a coherence theorem from which one should be able to extract a generators-and-relations description.

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  • $\begingroup$ Thanks to both of you. I"ll take a look at these things. $\endgroup$ Apr 25 '14 at 23:26
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    $\begingroup$ I realized later that there is a possible different answer to your second question, which might be more like what you had in mind. Namely, there are two more 2-categories $\Delta^\dagger$ and $\Delta^\diamond$, which have the analogous relationships to lax monads and oplax monads. They exist for formal reasons, but to my knowledge no one has studied them carefully or proven a coherence theorem. These really do "replace the simplicial identities by noninvertible 2-cells", whereas Todd's $\Delta$ keeps the simplicial identities as equalites and adds additional noninvertible 2-cells. $\endgroup$ Apr 26 '14 at 3:13
  • $\begingroup$ Yes, that is more like what I meant. In particular I was wondering about the lax version of "descent theory". $\endgroup$ Apr 26 '14 at 10:35
  • $\begingroup$ Now I discovered that Steve Lack's "Codescent objects and coherence, JPAA 175 (2002), 223–241" has some things about the lax situation. $\endgroup$ Apr 26 '14 at 16:19
  • $\begingroup$ Yes, that is also a good reference! $\endgroup$ Apr 26 '14 at 20:24
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In 2-category theory, there is a notion of "lax idempotent 2-monad" $M$ on a 2-category, for which the multiplication $m: M M \to M$ is left adjoint to the unit $u M: M \to M M$. A typical sort of example is a cocompletion monad with respect to some class of colimits. For example, consider the 2-category $Pos$ of posets; the 2-category $Sup$ of sup-lattices is 2-monadic over $Pos$, where the free sup-lattice generated by a poset $P$ is the sup-lattice of poset maps $P^{op} \to \mathbf{2} = \{0 \leq 1\}$ (equivalently, the sup-lattice of downward closed subsets). In this case the unit of the 2-monad is a Yoneda embedding $y_P: P \to [P^{op}, \mathbf{2}]$, and the 2-monad multiplication is

$$[y_P^{op}, \mathbf{2}]: [[P^{op}, \mathbf{2}]^{op}, \mathbf{2}] \to [P^{op}, \mathbf{2}].$$

What turns out to be true always is that $M u: M \to M M$ is in turn left adjoint to $m: M M \to M$.

Just as the augmented simplicial category $\Delta$ of finite ordinals (including the empty ordinal), as a monoidal 1-category equipped with a monoid = 1-element ordinal, is initial among monoidal categories equipped with a monoid, we can make an analogous statement for $\Delta$ as a 2-category (hom-sets being made into poset categories since ordinal maps are ordered among themselves). Namely, we can construct syntactically the initial monoidal 2-category equipped with a lax idempotent monoid, which will have invertible triangulator isomorphisms as part of the data, with suitable coherence conditions. And what turns out to be true is that this initial structure is equivalent (as a monoidal 2-category) to $\Delta$. Notice that 2-cell isomorphisms of $\Delta$ are automatically identities, meaning that the built-in coherence conditions on the initial structure can be "strictified".

The nLab has some information and references for lax idempotent monads which may be useful to you. I'll try to track down a reference for this 2-categorical fact about $\Delta$, but you can check by hand the presence of a lax idempotent (aka "Kock-Zöberlein") monoid structure on the 1-element ordinal.

Edit: Perhaps a canonical reference for this material is Street's Fibrations in Bicategories, particularly section 2.

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  • $\begingroup$ Thank you for the answer. This is interesting in its own way, but it does not quite answer the question. See Mike's comment below. $\endgroup$ Apr 26 '14 at 11:46
  • $\begingroup$ Fair enough. I wasn't quite clear on what you wanted. $\endgroup$
    – Todd Trimble
    Apr 26 '14 at 12:20

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