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Let $\mathbf{Top}$ be the category of (nice) topological spaces. For any space $Z$, define $\mathbf{End}_{\text{operad}}(Z)$ as the endomorphism operad.

Is there always a map of operads $$\mathbf{End}_{\text{operad}}(Z\times Z)\rightarrow \mathbf{End}_{\text{operad}}(Z) $$

Edit: Intuitively, I would say the answer should be NO i.e. there is NOT ALWAYS such a map of operads. But I don't have a concrete counterexample.

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  • $\begingroup$ Would you settle for a zigzag of maps instead of just one map? If so, the answer is yes in a few ways. $\endgroup$
    – donald yau
    Commented Jan 5, 2021 at 20:18

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The answer is no already for $Z = \mathbb{R}$. The endomorphism operad of $\mathbb{R}^2 \cong \mathbb{C}$ contains the Lawvere theory of $\mathbb{C}$-algebras, which cannot act on $\mathbb{R}$. This amounts to proving the following slightly strange statement:

No topological $\mathbb{C}$-algebra (meaning, for our purposes a $\mathbb{C}$-algebra equipped with a topology such that addition, multiplication, and scalar multiplication by a fixed $\lambda \in \mathbb{C}$ are all continuous; importantly, we don't require continuity in $\lambda$) can be homeomorphic to $\mathbb{R}$.

Proof. Suppose $A$ is a topological ring homeomorphic to $\mathbb{R}$. Then $(A, +)$ is a topological group homeomorphic to $\mathbb{R}$. By Gleason-Montgomery-Zippin's solution to Hilbert's fifth problem, $(A, +)$ is a $1$-dimensional simply connected Lie group, hence $(A, +) \cong (\mathbb{R}, +)$ as topological groups; from now on we will write $A = \mathbb{R}$.

(Edit: This is overkill, as expected. There is a totally elementary argument that a topological group (not necessarily abelian!) homeomorphic to $\mathbb{R}$ must be isomorphic to $(\mathbb{R}, +)$; see this math.SE question.)

Write $\cdot : \mathbb{R} \times \mathbb{R} \to \mathbb{R}$ for the multiplication on $A$. By rescaling we may assume WLOG that the multiplicative identity is $1$. It follows that $q_1 \cdot q_2 = q_1 q_2$ for rational $q_1, q_2$, and then by continuity it follows that $r_1 \cdot r_2 = r_1 r_2$ for all $r_1, r_2$, so $A \cong \mathbb{R}$ as a topological ring.

(So $(\mathbb{R}, +, \times)$ is the unique topological ring homeomorphic to $\mathbb{R}$, which is a fun fact that I don't think I've seen before.)

In particular, $A \cong \mathbb{R}$ as a ring, so admits no $\mathbb{C}$-algebra structure (even one where scalar multiplication may be discontinuous) because there is no ring homomorphism $\mathbb{C} \to \mathbb{R}$ (since $\mathbb{R}$ does not have an element satisfying $x^2 = -1$). $\Box$

Here I've blithely ignored the difference between a morphism of Lawvere theories and a morphism of operads, but I don't think it matters: concretely, the endomorphism operad of $\mathbb{R}^2 \cong \mathbb{C}$ contains binary operations describing addition and complex multiplication, and unary operations describing complex scalar multiplication, and nullary (or unary depending on taste) operations describing the additive and multiplicative identities, satisfying the $\mathbb{C}$-algebra axioms, and a morphism into another endomorphism operad $\textbf{End}(X)$ implies equipping $X$ with the same operations satisfying the same axioms, and that's all we need (even if there may or may not be other weird things going on involving the diagonal and projection maps).

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    $\begingroup$ Also, this is a lovely proof, and that fact about topological rings is indeed fun, but it's amazing to hit a fact about $\mathbb R$ with the full GMZ hammer! Do you know if one can streamline the proof in that special case? $\endgroup$
    – LSpice
    Commented Jan 6, 2021 at 0:18
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    $\begingroup$ @LSpice: good question! I haven't thought about it. I don't know much about the proof so I don't know if it substantially simplifies for $\mathbb{R}$. I guess the first thing I'd try is to move the identity to the origin and see if the group operation admits anything like a "Taylor expansion" near the origin? $\endgroup$ Commented Jan 6, 2021 at 0:24
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    $\begingroup$ @LSpice There's a structure theorem for LCA groups, whose details I never learned properly, but I think I've seen a more or less compete proof in the book of Deitmar and Echterhoff which doesn't need to invoke GMZ; I think somehow one can split off the non-Lie weird compact bits by hand using facts special to the Abelian group setting. If one used that structure theorem as a black box then I think one would end up showing that the original topological group had to be iso to ${\mathbb R}$ as a topological group, but I admit I haven't sat and thought through the details $\endgroup$
    – Yemon Choi
    Commented Jan 8, 2021 at 3:53
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    $\begingroup$ @Yemon: Oh, yes. Is it something like every LCA group is a direct product of $\mathbb{R}^n$ and an extension of a compact group by a discrete group? I guess it follows that a contractible LCA group is $\mathbb{R}^n$ for some $n$ (also a fun fact) and then we are done. $\endgroup$ Commented Jan 8, 2021 at 3:58
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    $\begingroup$ @LSpice: hmm, the argument I had in mind has a gap. A compact connected abelian group has Pontryagin dual a torsion-free discrete abelian group, which is a filtered colimit of $\mathbb{Z}^n$'s; so a compact connected abelian group is a cofiltered limit of tori $T^n$. What I would like to conclude at this point is that its fundamental group is the corresponding cofiltered limit of fundamental groups, but I don't actually see how to prove that. I don't think it's generally true that $\pi_1$ preserves cofiltered limits, unfortunately (is it?). $\endgroup$ Commented Jan 9, 2021 at 10:11

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