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Emily
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Grading ring spectra over the sphere spectrum

$\mathbb{Z}$-graded rings play an important role in algebra and algebraic geometry, so when moving to derived algebra and spectral algebraic geometry, it's natural to ask about ring spectra graded in the sphere spectrum $\mathbb{S}$.

As discussed in Section 2 of Ulrich–Nikolaus's Twisted differential cohomology, such an object may be defined as a symmetric lax monoidal functor $\mathbb{S}\to\mathsf{Sp}$, where $\mathbb{S}$ is considered as a monoidal $\infty$-groupoid.

In particular, given an $\mathbb{S}$-graded ring spectrum $R$, it follows that the ring $\pi_0(R)$ is $\mathsf{Ho}(\mathbb{S})$-graded. Since $\mathsf{Ho}(\mathbb{S})\cong\tau_{\leq1}(\mathbb{S})$, such a $\tau_{\leq1}\mathbb{S}$-graded ring consists of a pair $(R_\bullet,\{\sigma_k\}_{k\in\mathbb{Z}})$ with

  • $R_\bullet$ a $\mathbb{Z}$-graded ring;
  • $\{\sigma_k\colon R_k\to R_k\}_{k\in\mathbb{Z}}$ a family of order $2$ automorphisms, one for each $R_k$;

such that the component $$\mu_{n,m}\colon R_n\otimes R_m \to R_{n+m}$$ of the multiplication map of $R_\bullet$ at $(n,m)$ is commutative if $nm$ is even, and otherwise is so up to the automorphism $\sigma_{n+m}$ in that we have $$ ba = \begin{cases} ab &\text{if $nm$ is even,}\\ \sigma_{n+m}(ab) &\text{if $nm$ is odd} \end{cases} $$ for each $a\in R_n$ and each $b\in R_m$.

So, are there any interesting/non-trivial "in nature" examples of $\mathbb{S}$-graded ring spectra?

Emily
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