Suppose $X$ is a finite $n$-connected CW complex, then the function spectrum $F(\Sigma^\infty\_+ X,S^0)$ is an $E\_\infty$-ring spectrum, induced by the diagonal of $X$. I believe it is known that if we consider $F(\Sigma^\infty\_+ X,S^0)$ as an $E\_n$-ring spectrum, its Koszul dual is the $E\_n$-ring spectrum $\Sigma^\infty\_+ \Omega^n X$. Let me sketch an argument:


The Koszul dual $E\_n$-ring spectrum is computed as the Spanier-Whitehead dual of the factorization homology $\int\_{(\mathbb{R}^n)^+}(-)$. We know factorization homology commutes with taking suspension spectra, so
$\int\_{(\mathbb{R}^n)^+} \Sigma^\infty\_+ \Omega^n X \simeq \Sigma^\infty\_+ \int\_{(\mathbb{R}^n)^+} \Omega^n X$. By work of Ayala-Francis, the latter is known to be equivalent to $\Sigma^\infty\_+ B^n\Omega^n X\simeq \Sigma^\infty\_+ X$.


The tricky part is then to figure out the $E\_n$-coalgebra structure on the result. The $E\_n$-coalgebra structure is obtained from the functoriality of factorization homology with respect to pinch maps. Hence, the $E\_n$-coalgebra structure can be computed before taking suspension spectra. From [this question](https://mathoverflow.net/questions/434950/for-which-operads-o-does-operatornamecoalg-oc-c-whenever-c-is-carte), I learned that in a cartesian category like $(\mathrm{Top}\_\*,\times)$, all $E\_n$-coalgebras come from the diagonal, so the $E\_n$-coalgebra structure of $X$ induced from factorization homology has to be coming from the diagonal. The dual of this is then the original $E\_n$-algebra structure on $F(\Sigma^\infty\_+ X,S^0)$.


1. Is there a usual reference for this fact?
2. Is the supplied proof correct?



Koszul duals of n-fold loop spaces
I am not sure if this gives what you want, but maybe it is:
I went in the other direction in a paper *The McCord model for the tensor product of a space and a commutative ring spectrum,* in Progress in Math. **215** (2003).


Let $D(Y) = F(Y,S)$ denote the $S$-dual of a spectrum $Y$. Given a space $X$, $D(\Sigma^{\infty}\_+X)$ - let's write this as $D(X\_+)$ - is a commutative augmented $S$-algebra using the diagonal on $X$. The category of such things is tensored over based spaces, and I consider the augmented commutative algebra $S^n \otimes D(X\_+)$. (This can surely be viewed as factorization homology.)


This has an $E\_n$-coalgebra structure using the naturality in the $S^n$ variable, and so the $S$--dual, $D(S^n \otimes D(X\_+))$ is an $E\_n$-algebra. In my paper, I observe that when $X$ is finite and $n$-connected, this $E\_n$-algebra identifies with the $E\_n$-algebra $\Sigma^{\infty}\_+ \Omega^n X$. (I prove a more general result, and compare with a paper of Greg Arone. I show that certain Goodwillie towers he wrote down are obtained by taking the $S$--dual of filtered objects that I construct.)


