superalgebra

and

supergeometry

# Contents

## Definition

The odd line is the supermanifold $\mathbb{R}^{0|1}$ – a super Cartesian space and in particular a superpoint – characterized by the fact that its $\mathbb{Z}_2$-graded algebra of functions is the algebra free on a single odd generator $\theta$: $C^\infty(\mathbb{R}^{0|1}) = \mathbb{R}[\theta] = \mathbb{R} \oplus \theta\cdot \mathbb{R}$.

This algebra is essentially the ring of dual numbers, but with the single generator in odd degree.

## Properties

### The automorphism super-group

The interal automorphism group of the odd line in the topos of smooth super spaces is the supergroup

$\mathbf{Aut}(\mathbb{A}^{0|1}) \simeq \mathbb{G}_m \ltimes (\Pi \mathbb{G}_{ad})$

which is the semidirect product group of the multiplicative group (the group of units, hence $\mathbb{R}^\times$ when working over the real numbers) with the additive group shifted into odd degree. (See at References – Automorphism group for the origin of this observation.)

In the topos over superpoints $\mathbb{R}^{0|q}$ this is seen over the test space $\mathbb{R}^{0|1}$ itself with canonical odd coordinate $\theta$ by taking the canonical odd coordinate of the odd line that we are taking automorphism of to be $\epsilon$ and observing that maps

$\mathbb{R}^{0|1} \to \mathbf{Aut}(\mathbb{R}^{0|1})$

are then given, under the evaluation map-isomorphism and via the Yoneda lemma by Grassmann algebra homomorphisms of the form

$\langle \epsilon, \theta\rangle \leftarrow \langle \epsilon\rangle$

that send

$\epsilon \mapsto x \epsilon + y \theta$

with $x \neq 0$ in even degree and arbitrary $y$ in odd degree. Notice that the inverse of this map exists and is given by

$\epsilon \mapsto x^{-1}\epsilon - y x^{-1} \theta \,.$

Moreover, observe that an action of $\mathbf{Aut}(\mathbb{R}^{0|1})$ on a supermanifold corresponds to a choice of grading and a choice of differential. A clean account of this statement is in (Carchedi-Roytenberg 12). (At least for the grading this is essentially the classical statement about graded rings, see at affine line the section Properties – Grading).

## References

### Automorphism group

That an action of the endomorphism/automorphism supergroup of the odd line on a supermanifold is equivalent to a choice of grading and a differential first observed in

It was later amplified in section 3.2 of

where it is used to exhibit the canonical de Rham differential action on the odd tangent bundle? $Maps(\mathbb{R}^{0|1}, X)$ of a supermanifold $X$.

The same mechanism is amplified further in the discussion of derived differential geometry in

An interpretation of an $\mathbb{G}_m \ltimes \Pi \mathbb{G}_{ad}$-action on a supermanifold of local quantum observables of a supersymmetric field theory as the formalization of the concept of topologically twisted super Yang-Mills theory is in section 15 of

For more on this see at topologically twisted super Yang-Mills theory – Formalization.

Revised on May 14, 2015 08:53:10 by Urs Schreiber (195.113.30.252)