5
$\begingroup$

Suppose that $\mathcal{A}, \mathcal{B}$ are strictly unital $A_\infty$ categories, and $\mathcal{F}, \mathcal{G}: \mathcal{A} \rightarrow \mathcal{B}$ are (strictly unital) functors.

On one hand, we can say that $\mathcal{F}$ and $\mathcal{G}$ are quasi-equivalent if the honest functors $H(\mathcal{F}), H(\mathcal{G}): H(\mathcal{A}) \rightarrow H(\mathcal{B})$ agree. (Perhaps, morally, I want to write 'up to natural isomorphism' here.)

On the other, I can consider $\mathcal{F}$ and $\mathcal{G}$ as objects of the functor category $\sf{fun}(\mathcal{A}, \mathcal{B})$. Here, I say that $\mathcal{F}$ and $\mathcal{G}$ are homotopy equivalent if they are isomorphic as elements of $H(\sf{fun}(\mathcal{A}, \mathcal{B}))$; to spell this out, this is if there are natural transformations $S: \mathcal{F} \Rightarrow \mathcal{G}$ and $T: \mathcal{G} \Rightarrow \mathcal{F}$, a pre-natural transformation $P: \mathcal{F} \Rightarrow \mathcal{F}$ with $$\mu^1_{\sf{fun}(\mathcal{A}, \mathcal{B})}(P) = T \circ S - \mathrm{Id}_F,$$ and similar for $S \circ T$, via a pre-natural transformation $Q$.

Is it true that if $\mathcal{F}$ and $\mathcal{G}$ are quasi-equivalent then they are homotopy equivalent?

If the answer is 'not in general', are there conditions upon $\mathcal{B}$ which make it so? I think, if I understand the literature, that if $\mathcal{B}$ is the (dg) category of chain complexes so that the functor category is $\sf{Mod}_{\mathcal{A}^{\mathrm{opp}}}$ then the answer is 'yes' - perhaps provided that the ground ring is a field of characteristic two?

(This is, clearly, a somewhat naiive question to ask; I ask it because of the general phenomenon in $A_{\infty}$ categories that often quasi-somethings are also homotopy-somethings, usually via some sort of perturbation theory argument - I can't see anything to the effect of the above in, e.g. Seidel's book, but I also lose my marbles a little moving between the relevant categories and so could quite easily have overlooked something.)

$\endgroup$

2 Answers 2

8
$\begingroup$

As you expect, the answer is negative in general, although counterexamples are relatively recent. You can find a counterexample in:

Alberto Canonaco y Paolo Stellari, «Non-Uniqueness of Fourier–Mukai Kernels», Mathematische Zeitschrift 272, n.º 1 (1 de octubre de 2012): 577-88, https://doi.org/10.1007/s00209-011-0950-3.

There, it's stated in terms of Fourier-Mukai functors between derived categories of elliptic curves. However, it's equivalent to your question. This is explained in:

Francesco Genovese, «The Uniqueness Problem of Dg-Lifts and Fourier-Mukai Kernels», Journal of the London Mathematical Society 94, n.º 2 (2016): 617-38, https://doi.org/10.1112/jlms/jdw052.

There, you also find sufficient conditions to get a positive answer to your question.

$\endgroup$
1
$\begingroup$

Take a formal diffeomorphism of $n$-dimensional affine space, meaning a change of variables given by formal power series $(f_1(x_1,\dots,x_n),\dots,f_n(x_1,\dots,x_n))$ preserving the origin. This preserves the structure sheaf at the origin (the one-dimensional simple module over the power series ring); hence it induces an $A_\infty$-automorphism of the $Ext$ algebra of that sheaf, which is an exterior algebra.

Here, you can see the discrepancy very clearly: the action on $H^*(A)$ only gives the Jacobian of the diffeomorphism, whereas the $A_\infty$-automorphism structure recovers the higher order terms.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .