Simplicial sets are presheaves on the simplex category $\Delta$, while augmented simplicial sets are presheaves on $\Delta\_+$, the augmented simplex category. Because Day convolution allows us to lift monoidal structures on a category $\mathcal{C}$ to its category of presheaves $\mathrm{Sets}^{\Delta^\circ}$, it is therefore of interest to find monoidal structures on $\Delta$ and $\Delta\_+$, as these then provide "natural" monoidal structures on simplicial sets.


The only monoidal structure I know of is the ordinal sum of $\Delta\_+$ (which is not braided), whose Day convolution gives the join of simplicial sets, and whose internal hom is given by
$$[X,Y]\_n=\mathrm{hom}\_{\mathrm{Sets}^{\Delta^\circ\_+}}(X,\mathrm{Dec}^{n+1}Y)$$


Is this the only monoidal structure on $\Delta\_+$? If not, what other monoidal structures are there on $\Delta\_+$, and what are there on $\Delta$?



Is $\oplus$ the only monoidal structure on the simplex category?
Here is half of a classification. Let $\otimes$ be a monoidal structure on $\Delta\_+$. As I mentioned in a [comment](https://mathoverflow.net/questions/402257/is-oplus-the-only-monoidal-structure-on-the-simplex-category#comment1032524_402257), the monoidal unit must be $[-1]$ or $[0]$ because these are the only objects with commutative endomorphism monoids.


Suppose that the monoidal unit is $[0]$. Let us consider $[1] \otimes [1]$. We have that $[0]$ is a retract of $[1]$ in 2 ways, and as a result we obtain 4 retracts of $[1] \otimes [1]$ with support $[0] \otimes [0] = [0]$. Consider the induced linear ordering on these 4 points. We also have 4 ways that $[1] = [1] \otimes [0] = [0] \otimes [1]$ is a retract of $[1] \otimes [1]$, and from this we can deduce most of the ordering. It must have the following relations


$\require{AMScd} \begin{CD} 0 \otimes 0 @>>> 0 \otimes 1\\ @VVV @VVV\\ 1 \otimes 0 @>>> 1 \otimes 1 \end{CD}$


To complete this to a linear order, without loss of generality we must have $0 \otimes 1 \leq 1 \otimes 0$. But now, one of our 4 projections onto $[1]$ is the coordinate projection onto the right column of the above square. The fact that this projection is order-preserving implies that $1 \otimes 0 = 0 \otimes 1 = 1 \otimes 1$. This contradicts the fact that the right column exhibits $[1]$ as a retract of this subset of $[1] \otimes [1]$.


Therefore the monoidal unit is not $[0]$; it must be $[-1]$.


I also think I'm ready to conjecture that $\oplus$, $\oplus^{rev}$ (as mentioned [by Peter](https://mathoverflow.net/questions/402257/is-oplus-the-only-monoidal-structure-on-the-simplex-category?noredirect=1#comment1032530_402257)) and the [degenerate monoidal structure](https://mathoverflow.net/questions/402257/is-oplus-the-only-monoidal-structure-on-the-simplex-category?noredirect=1#comment1032531_402257) are probably the only ones. You could imagine a classification starting as follows. Consider the maps $[0] = [0] \otimes [-1] \to [0] \otimes [0]$ and $[0] = [-1] \otimes [0] \to [0] \otimes [0]$. If these are the same, then we should have the degenerate monoidal structure. If they are different, then one is less than the other, and those two cases should correspond to $\oplus$ and $\oplus^{rev}$.


