$\renewcommand{\S}{\mathcal{S}}\newcommand{\l}{\langle}\newcommand{\r}{\rangle}\newcommand{\op}{\mathsf{op}}\newcommand{\fin}{\mathrm{fin}}$Recently I've noticed that the definitions of special $\Gamma$-spaces and spectra are quite close in spirit:
- $\Gamma$-spaces are pointed functors $X\colon(\Gamma^\op,\l0\r)\to(\mathcal{S},*)$ from Segal's category to the category $\mathcal{S}$ of spaces. Moreover, we call $X$
- special if $X$ sends coproducts to products;
- very special if $\pi_0(X_{\l1\r})$ is a group.
- Spectra are reduced excisive functors $E\colon\mathcal{S}^\fin_*\to\S$ of $\infty$-categories, where
- $E$ is excisive if it sends pushouts to pullbacks.
- $E$ is reduced if $E(*)\simeq *$;
Since very special $\Gamma$-spaces are equivalent to connective spectra, this made me wonder: can we view nonconnectivity as arising from enlarging Segal's category $\Gamma^{\mathsf{op}}\overset{\mathrm{def}}{=}\mathsf{Sk}(\mathsf{FinSets}_*)$ of finite pointed sets into the $\infty$-category $\S^\fin_*$ of finite pointed spaces?
In particular, two natural intermediate steps to consider between $\Gamma^{\mathsf{op}}$ and $\S^\fin_*$ are, for each $n\in\mathbb{N}$, the $\infty$-categories $\S^{\fin}_{\leq n,*}$ and $\S^{\fin}_{>n,*}$ of $n$-truncated and $n$-connected spaces, which come with natural inclusions of $\infty$-categories $\iota_{1},\iota_{2}\colon\S^{\fin}_{\leq n,*},\S^{\fin}_{>n,*}\hookrightarrow\S^\fin_*$.
Questions:
- Can we describe functors $E\colon\S^{\fin}_{\leq n,*}\hookrightarrow\S$ or $E\colon\S^{\fin}_{>n,*}\hookrightarrow\S$ in terms of spectra? Are the former maybe somehow related to $(-n-1)$-connected spectra?
- Since $\mathcal{S}$ is co/complete, we can consider left and right Kan extensions along $\iota_{1}$ and $\iota_{2}$, giving functors of the form $$\mathrm{Lan}_{\iota_{1}}\colon\mathsf{Exc}_*(\mathcal{S}^\fin_{\leq n,*})\to\mathsf{Sp}.$$ What are the essential images of $\mathrm{Lan}_{\iota_{1}}$, $\mathrm{Lan}_{\iota_{2}}$, $\mathrm{Ran}_{\iota_{1}}$, and $\mathrm{Ran}_{\iota_{2}}$?
Lastly, let me mention that this was asked also by Jonathan on the Homotopy Theory Discord. There, Rune Haugseng mentioned this paper of Harpaz, where one finds a related construction. Harpaz uses spans of $n$-finite spaces (meaning $n$-truncated spaces which additionally have finite homotopy groups), formulating a notion of $n$-commutative monoid for finite $n$ (which has since been extended to the $n=\infty$ case by Carmeli–Schlank–Yanovski). These are related to $n$-semiadditivity, and extend the commutativity ladder $\mathbb{E}_{1}$, $\mathbb{E}_{2}$, $\ldots$, $\mathbb{E}_{\infty}$ of monoid objects in an $\infty$-category in such a way that
- A $(-2)$-commutative monoid is precisely an $\mathbb{E}_{-1}$-monoid (where $\mathbb{E}_{-1}\overset{\mathrm{def}}{=}\mathsf{Triv}^\otimes$), meaning just an object;
- A $(-1)$-commutative monoid is precisely an $\mathbb{E}_{0}$-monoid, meaning a pointed object;
- A $0$-commutative monoid is precisely an $\mathbb{E}_{\infty}$-monoid.
One could imagine also more generally expanding the definition of $\infty$-operad, replacing $\mathrm{N}_{\bullet}(\mathsf{Fin}_*)$ by $\S^\fin_*$ and variants. For spans in $m$-finite spaces, this should give a notion of $m$-operad, as pointed out by Shachar Carmeli on the homotopy Discord.