I'm not very familiar with dg algebras (not necessarily commutative) and I'm wondering if any $E\_1$ algebra in the sense of infinity categories (i.e. monoid in the stable category of $R-Mod$) over $R$ some commutative ring (discrete) can be described by a dg algebra over R? Specifically I have some $E\_1$ algebra in mind which has homotopy groups $\mathbb{F}\_p$ in homological degrees $0$ and $-1$. I'm wondering if the $E\_1$ structure has to be the "obvious" one. I think there may be other $E\_1$ structures on it but I'm not sure how to see this.



Can any $E_1$ algebra over $\mathbb{F}_p$ be modeled as a dg algebra?
Yes, this is precisely the content of Theorem 7.11 in [arXiv:1410.5675](https://arxiv.org/abs/1410.5675), which should be combined with §7.4 of [arXiv:1510.04969](https://arxiv.org/abs/1510.04969).
In fact, the cited results prove this for any nonsymmetric operad in chain complexes over a commutative ring,
and are also applicable to ∞-categories other than chain complexes.


In the case of symmetric operads (such as ${\rm E}\_∞$) the result continues to hold
for characteristic 0, i.e., rational chain complexes.
For symmetric operads in characteristic $p$ there are algebras that cannot be rectified as stated,
and one must use other categories instead of chain complexes,
e.g., simplicial modules over ${\bf F}\_p$ (see §7.3 of [arXiv:1510.04969](https://arxiv.org/abs/1510.04969)).


In all such results, there are two main ingredients.
The first ingredient shows that any ∞-algebra over an ∞-operad $O$
can be rectified to a strict algebra over a strict operad $Q$,
where the underlying ∞-operad of $Q$ is equivalent to $O$.
This roughly amounts to the free strict $Q$-algebra
computing the free ∞-algebra over $O$,
which in its turn almost immediately boils down to
the strict coinvariants of tensor products $Q\_n⊗\_{Σ\_n}X^{⊗n}$ computing the corresponding homotopy coinvariants of derived tensor products,
for any cofibrant object $X$.
The main property required here is that $Q\_n$ should be cofibrant
(projectively cofibrant with respect to the $Σ\_n$-action in the case of symmetric operads).


The second ingredient shows that any strict $Q$-algebra
can be rectified to a strict $R$-algebra,
where $R$ is the operad we are interested in, e.g., the associative or commutative operad,
and $f\colon Q→R$ is a weak equivalence of operads.
The main property required here is that $f\_n ⊗\_{Σ\_n} X^{⊗n}$
should be a weak equivalence for any cofibrant object $X$.
This can happen for two reasons:
either the maps $f\_n$ are sufficiently nice
(e.g., their source and target are $Σ\_n$-projectively cofibrant)
or the category in which we work is nice.
The latter holds, for example, for rational chain complexes
and for various categories of symmetric spectra.


For nonsymmetric operads, such as the associative operad,
we can drop $Σ\_n$, which makes the condition much easier to satisfy
in practice.
In particular, it is always true for simplicial sets, chain complexes over any commutative ring, topological spaces, simplicial modules, simplicial presheaves, etc.
In all these cases, ∞-algebras over nonsymmetric operads
can always be rectified to strict algebras.


