# nLab formal moduli problem

### Context

#### Small objects

objects $d \in C$ such that $C(d,-)$ commutes with certain colimits

## Constructions

• fundamental ∞-groupoid in a locally ∞-connected (?,1)-topos? / of a locally ∞-connected (?,1)-topos?

# Contents

## Definition

###### Definition

Given a deformation context $(\mathcal{Y}, \{E_\alpha\}_\alpha)$, the (?,1)-category? of formal moduli problems over it is the full sub-(?,1)-category? of the (?,1)-category of (?,1)-presheaves? over $\mathcal{Y}^{inf}$

$Moduli^\mathcal{Y} \hookrightarrow [\mathcal{Y}^{inf}, \infty Grpd]$

on those (?,1)-functors? $X \colon \mathcal{Y}^{inf} \to \infty Grpd$ such that

1. over the terminal object they are contractible: $X(*) \simeq *$ (hence they are anti-reduced);

2. they preserve (?,1)-pullbacks? (are infinitesimally cohesive?)

###### Remark

This means that a “formal deformation problem” is a space in higher geometry whose geometric structure is detected by the “test spaces” in $\mathcal{Y}^{op}$ in a way that respects gluing (descent) in $\mathcal{Y}^{op}$ as given by (?,1)-pullbacks? there. The first condition requires that there is an essentially unique such probe by the point, hence that these higher geometric space has essentially a single global point. This is the condition that reflects the infinitesimal nature of the deformation problem.

## Properties

### Relation to $L_\infty$-algebras

For $k$ a field of characteristic 0, write write $CAlg_k^{sm} \hookrightarrow CAlg_k$ for the (?,1)-category? of Artinian connective E-? algebras? over $k$, or equivalently that of “small” commutative dg-algebras over $k$.

The smallness condition implies connectivity (Lurie, prop. 1.1.11 (1)), hence that the homotopy group of these E-? algebras? vanish in negative degree. Notice that for the dg-algebras this means that the chain homology vanishes in negative degree if the differential is taken to have degree -1 (see Porta 13, def. 3.1.14 for emphasis). This is the natural condition for the function algebra in derived geometry. Here these small $E_\infty$/dg-algebras are to be thought of as function algebras on “derived infinitesimally thickened points”.

###### Definition

There is an equivalence of (?,1)-categories?

$L_\infty Alg_k \stackrel{\simeq}{\to} Moduli^{CAlg^{sm}_k}$

with that of L-? algebras?.

In this form this is (Lurie, theorem 0.0.13). See at model structure for L-? algebras? for various other incarnations of this equivalence.

### Relation to Lie differentiation

###### Proposition

Given a deformation context $\mathcal{Y}$, the restricted (?,1)-Yoneda embedding? gives an (?,1)-functor?

$Lie \colon \mathcal{Y} \to Moduli^{\mathcal{Y}} \,.$
###### Remark

For $Y \in \mathcal{Y}^{op}$, the object $Lie(Y)$ represents the formal neighbourhood of the basepoint of $Y$ as seen by the infinitesimally thickened points dual to the $\{E_\alpha\}$.

Hence we may call this the operaton of Lie differentiation of spaces in $\mathcal{Y}^{op}$ around their given base point.

## References

• Jacob Lurie Moduli problems for ring spectra ICM 2010 proceedings contribution pdf

• Mauro Porta, Derived formal moduli problems, master thesis 2013, pdf.

Revised on October 26, 2016 14:25:49 by Urs Schreiber (89.204.155.11)