(ofc you should be careful, bc in real life you could have, like, m₁=m₂=r=5 and then you'll get confused) ⌊06⌋
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Define γ(i,r)=m₁+⋯+mᵢ and γ(k,mᵢ)=γ(i-1,r)+k. Here's a picture of where the images of the first few elements land inside of [Σmᵢ]. ⌊07⌋pic.twitter.com/bew6poaFAF
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To see that this is an operad, one must check that the "two ways" of getting from [r] ⨿ (∐_{i=1}^r [mᵢ] ⨿ (∐_{j=1}^{mᵢ} [nᵢⱼ])) to [Σnᵢⱼ] are the same. ⌊08⌋pic.twitter.com/3XKmMFPros
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Are you satisfied that the standard simplices form an operad? If so, let X be a simplicial set. We know that hom(A⨿B,X) = hom(A,X)×hom(B,X) and X(n) = hom(Δⁿ,X). (here, hom means "simplicial set morphisms", while earlier it meant morphisms in 𝚫) ⌊09⌋
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It follows that the maps γ from above give functions X(Σmᵢ) → X(r)×(X(m₁)×⋯×X(mᵣ)) and the unique map ∅ → [1] gives X(1) → hom(∅,X) = ∗. ⌊10⌋
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Because the maps γ were not injective, the cocomposition map X(Σmᵢ) → X(r)×(X(m₁)×⋯×X(mᵣ)) lands in a smaller subset. ⌊11⌋
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More specifically, this is an X(1)-colored cooperad. The "output color" of an element x ∊ X(n) is found using the endpoint preserving function [1] → [n] in 𝚫 (that is, 0↦0, 1↦n). ⌊12⌋
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For 1≤j≤n, the jth "input color" of x∊X(n) is given by the interval inclusion [1] → [n] in 𝚫 that sends 0 to j-1 and 1 to j. ⌊13⌋
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For each simplicial set X, we have produced an X(1)-colored cooperad [in (Set,×,∗)]. ⌊14⌋
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If X happens to be "2-Segal" then this cooperad is "invertible". This means we can reverse the structure maps (which are bijections), and so our simplicial set is also an operad. ⌊15⌋
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solifine je proslijedio/a tweet korisnika/cesolifine
I talked about an example of this before, namely when X is the nerve of a category. More can be found in §3.6 of Dyckerhoff–Kapranov's "Higher Segal Spaces". ⌊16⌋https://twitter.com/solifine/status/1213137923870208000 …
solifine je dodan/na,
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