# Diffeology

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In mathematics, a diffeology on a set generalizes the concept of smooth charts in a differentiable manifold, declaring what the "smooth parametrizations" in the set are.

The concept was first introduced by Jean-Marie Souriau in the 1980s under the name Espace différentiel[1][2] and later developed by his students Paul Donato[3] and Patrick Iglesias.[4][5] A related idea was introduced by Kuo-Tsaï Chen (陳國才, Chen Guocai) in the 1970s, using convex sets instead of open sets for the domains of the plots.[6]

## Intuitive definition

Recall that a topological manifold is a topological space which is locally homeomorphic to ${\displaystyle \mathbb {R} ^{n}}$. Differentiable manifolds generalize the notion of smoothness on ${\displaystyle \mathbb {R} ^{n}}$ in the following sense: a differentiable manifold is a topological manifold with a differentiable atlas, i.e. a collection of maps from open subsets of ${\displaystyle \mathbb {R} ^{n}}$ to the manifold which are used to "pull back" the differential structure from ${\displaystyle \mathbb {R} ^{n}}$ to the manifold.

A diffeological space consists of a set together with a collection of maps (called a diffeology) satisfying suitable axioms, which generalise the notion of an atlas on a manifold. In this way, the relationship between smooth manifolds and diffeological spaces is analogous to the relationship between topological manifolds and topological spaces:

More precisely, a smooth manifold can be equivalently defined as a diffeological space which is locally diffeomorphic to ${\displaystyle \mathbb {R} ^{n}}$. Indeed, every smooth manifold has a natural diffeology, consisting of its maximal atlas (all the smooth maps from open subsets of ${\displaystyle \mathbb {R} ^{n}}$ to the manifold). This abstract point of view makes no reference to a specific atlas (and therefore to a fixed dimension ${\displaystyle n}$) nor to the underlying topological space, and is therefore suitable to treat examples of objects more general than manifolds.

## Formal definition

A diffeology on a set ${\displaystyle X}$ consists of a collection of maps, called plots or parametrizations, from open subsets of ${\displaystyle \mathbb {R} ^{n}}$ (${\displaystyle n\geq 0}$) to ${\displaystyle X}$ such that the following properties hold:

• Every constant map is a plot.
• For a given map, if every point in the domain has a neighborhood such that restricting the map to this neighborhood is a plot, then the map itself is a plot.
• If ${\displaystyle p}$ is a plot, and ${\displaystyle f}$ is a smooth function from an open subset of some real vector space into the domain of ${\displaystyle p}$, then the composition ${\displaystyle p\circ f}$ is a plot.

Note that the domains of different plots can be subsets of ${\displaystyle \mathbb {R} ^{n}}$ for different values of ${\displaystyle n}$; in particular, any diffeology contains the elements of its underlying set as the plots with ${\displaystyle n=0}$. A set together with a diffeology is called a diffeological space.

More abstractly, a diffeological space is a concrete sheaf on the site of open subsets of ${\displaystyle \mathbb {R} ^{n}}$, for all ${\displaystyle n\geq 0}$, and open covers.[7]

### Morphisms

A map between diffeological spaces is called differentiable (or smooth) if and only if its composition with any plot of the first space is a plot of the second space. It is called a diffeomorphism if it is differentiable, bijective, and its inverse is also differentiable.

Diffeological spaces form a category, whose morphisms are differentiable maps. The category of diffeological spaces is closed under many categorical operations: for instance, it is Cartesian closed, complete and cocomplete, and more generally it is a quasitopos.[7]

### Additional structures

Any diffeological space is automatically a topological space with the so-called D-topology: the finest topology such that all plots are continuous (with respect to the euclidean topology on ${\displaystyle \mathbb {R} ^{n}}$). A differentiable map between diffeological spaces is automatically continuous between their D-topologies.

A Cartan-De Rham calculus can be developed in the framework of diffeology, as well as a suitable adaptation of the notions of fiber bundles, homotopy, etc.[5] However, there is not a canonical definition of tangent spaces and tangent bundles for diffeological spaces.[8]

## Examples

### Manifolds

• Any differentiable manifold is a diffeological space together with its maximal atlas (i.e., the plots are all smooth maps from open subsets of ${\displaystyle \mathbb {R} ^{n}}$ to the manifold); its D-topology recovers the original manifold topology. With this diffeology, a map between two smooth manifolds is smooth if and only if it is differentiable in the diffeological sense. Accordingly, smooth manifolds with smooth maps form a full subcategory of the category of diffeological spaces.
• Similarly, complex manifolds, analytic manifolds, etc. have natural diffeologies consisting of the maps preserving the extra structure.
• This method of modeling diffeological spaces can be extended to locals models which are not necessarily the euclidean space ${\displaystyle \mathbb {R} ^{n}}$. For instance, diffeological spaces include orbifolds, which are modeled on quotient spaces ${\displaystyle \mathbb {R} ^{n}/\Gamma }$, for ${\displaystyle \Gamma }$ is a finite linear subgroup,[9] or manifolds with boundary and corners, modeled on orthants, etc.[10]
• Any Banach manifold is a diffeological space.[11]
• Any Fréchet manifold is a diffeological space.[12][13]

### Constructions from other diffeological spaces

• If ${\displaystyle Y}$ is a subset of the diffeological space ${\displaystyle X}$, then the subspace diffeology on ${\displaystyle Y}$ is the diffeology consisting of the plots of ${\displaystyle X}$ whose images are subsets of ${\displaystyle Y}$. The D-topology of ${\displaystyle Y}$ is the subspace topology of the D-topology of ${\displaystyle X}$.
• If ${\displaystyle X}$ and ${\displaystyle Y}$ are diffeological spaces, then the product diffeology on the Cartesian product ${\displaystyle X\times Y}$ is the diffeology generated by all products of plots of ${\displaystyle X}$ and of ${\displaystyle Y}$. The D-topology of ${\displaystyle X\times Y}$ is the product topology of the D-topologies of ${\displaystyle X}$ and ${\displaystyle Y}$.
• If ${\displaystyle X}$ is a diffeological space and ${\displaystyle \sim }$ is an equivalence relation on ${\displaystyle X}$, then the quotient diffeology on the quotient set ${\displaystyle X}$/~ is the diffeology generated by all compositions of plots of ${\displaystyle X}$ with the projection from ${\displaystyle X}$ to ${\displaystyle X/\sim }$. The D-topology on ${\displaystyle X/\sim }$ is the quotient topology of the D-topology of ${\displaystyle X}$ (note that this topology may be trivial without the diffeology being trivial).
• The pushforward diffeology of a diffeological space ${\displaystyle X}$ by a function ${\displaystyle f:X\to Y}$ is the diffeology on ${\displaystyle Y}$ generated by the compositions ${\displaystyle f\circ p}$, for ${\displaystyle p}$ a plot of ${\displaystyle X}$. In other words, the pushforward diffeology is the smallest diffeology on ${\displaystyle Y}$ making ${\displaystyle f}$ differentiable. The quotient diffeology boils down to the pushforward diffeology by the projection ${\displaystyle X\to X/\sim }$.
• The pullback diffeology of a diffeological space ${\displaystyle Y}$ by a function ${\displaystyle f:X\to Y}$ is the diffeology on ${\displaystyle X}$ whose plots are maps ${\displaystyle p}$ such that the composition ${\displaystyle f\circ p}$ is a plot of ${\displaystyle X}$. In other words, the pullback diffeology is the smallest diffeology on ${\displaystyle X}$ making ${\displaystyle f}$ differentiable.
• The functional diffeology between two diffeological spaces ${\displaystyle X,Y}$ is the diffeology on the set ${\displaystyle {\mathcal {C}}^{\infty }(X,Y)}$ of differentiable maps, whose plots are the maps ${\displaystyle \phi :U\to {\mathcal {C}}^{\infty }(X,Y)}$ such that ${\displaystyle (u,x)\mapsto \phi (u)(x)}$ is smooth (with respect to the product diffeology of ${\displaystyle U\times X}$). When ${\displaystyle X}$ and ${\displaystyle Y}$ are manifolds, the D-topology of ${\displaystyle {\mathcal {C}}^{\infty }(X,Y)}$ is the smallest locally path-connected topology containing the weak topology.[14]

### More general examples

• Any set can be endowed with the coarse (or trivial, or indiscrete) diffelogy, i.e. the largest possible diffeology (any map is a plot). The corresponding D-topology is the trivial topology.
• Any set can be endowed with the discrete (or fine) diffeology, i.e. the smallest possible diffeology (the only plots are the locally constant maps). The corresponding D-topology is the discrete topology.
• Any topological space can be endowed with the continuous diffeology, whose plots are all continuous maps. The corresponding D-topology is of course the original topology of the space.
• Quotients gives an easy way to construct non-manifold diffeologies. For example, the set of real numbers ${\displaystyle \mathbb {R} }$ is a smooth manifold. The quotient ${\displaystyle \mathbb {R} /(\mathbb {Z} +\alpha \mathbb {Z} )}$, for some irrational ${\displaystyle \alpha }$, called irrational torus, is a diffeological space diffeomorphic to the quotient of the regular 2-torus ${\displaystyle \mathbb {R} ^{2}/\mathbb {Z} ^{2}}$ by a line of slope ${\displaystyle \alpha }$. It has a non-trivial diffeology, but its D-topology is the trivial topology.[15]
• Combining the subspace diffeology and the functional diffeology, one can define diffeologies on the space of sections of a fibre bundle, or the space of bisections of a Lie groupoid, etc.

## Subductions and inductions

Analogously to the notions of submersions and immersions between manifolds, there are two special classes of morphisms between diffeological spaces. A subduction is a surjective function ${\displaystyle f:X\to Y}$ between diffeological spaces such that the diffeology of ${\displaystyle Y}$ is the pushforward of the diffeology of ${\displaystyle X}$. Similarly, an induction is an injective function ${\displaystyle f:X\to Y}$ between diffeological spaces such that the diffeology of ${\displaystyle X}$ is the pullback of the diffeology of ${\displaystyle Y}$. Note that subductions and inductions are automatically smooth.

When ${\displaystyle X}$ and ${\displaystyle Y}$ are smooth manifolds, a subduction (respectively, induction) between them is precisely a surjective submersion (respectively, injective immersion). Moreover, these notions enjoy similar properties to submersion and immersions, such as:

• A composition ${\displaystyle f\circ g}$ is a subduction (respectively, induction) if and only if ${\displaystyle f}$ is a subduction (respectively, ${\displaystyle g}$ is an induction).
• An injective subduction (respectively, a surjective induction) is a diffeomorphism.

Last, an embedding is an induction which is also a homeomorphism with its image, with respect to the subset topology induced from the D-topology of the codomain. This boils down to the standard notion of embedding between manifolds.

## References

1. ^ Souriau, J. M. (1980), García, P. L.; Pérez-Rendón, A.; Souriau, J. M. (eds.), "Groupes differentiels", Differential Geometrical Methods in Mathematical Physics, Berlin, Heidelberg: Springer Berlin Heidelberg, 836, pp. 91–128, doi:10.1007/bfb0089728, ISBN 978-3-540-10275-5, retrieved 2022-01-16
2. ^ Souriau, Jean-Marie (1984), Denardo, G.; Ghirardi, G.; Weber, T. (eds.), "Groupes différentiels et physique mathématique", Group Theoretical Methods in Physics, Berlin/Heidelberg: Springer-Verlag, 201, pp. 511–513, doi:10.1007/bfb0016198, ISBN 978-3-540-13335-3, retrieved 2022-01-16
3. ^ Donato, Paul (1984). Revêtement et groupe fondamental des espaces différentiels homogènes [Coverings and fundamental groups of homogeneous differential spaces] (in French). Marseille: PhD thesis, Université de Provence.
4. ^ Iglesias, Patrick (1985). Fibrés difféologiques et homotopie [Diffeological fiber bundles and homotopy] (PDF) (in French). Marseille: PhD thesis, Université de Provence.
5. ^ a b Iglesias-Zemmour, Patrick (2013-04-09). Diffeology. Mathematical Surveys and Monographs. 185. American Mathematical Society. doi:10.1090/surv/185. ISBN 978-0-8218-9131-5.
6. ^ Chen, Kuo-Tsai (1977). "Iterated path integrals". Bulletin of the American Mathematical Society. 83 (5): 831–879. doi:10.1090/S0002-9904-1977-14320-6. ISSN 0002-9904.
7. ^ a b Baez, John; Hoffnung, Alexander (2011). "Convenient categories of smooth spaces". Transactions of the American Mathematical Society. 363 (11): 5789–5825. doi:10.1090/S0002-9947-2011-05107-X. ISSN 0002-9947.
8. ^ Christensen, Daniel; Wu, Enxin (2016). "Tangent spaces and tangent bundles for diffeological spaces". Cahiers de Topologie et Geométrie Différentielle Catégoriques. 57 (1): 3–50.
9. ^ Iglesias-Zemmour, Patrick; Karshon, Yael; Zadka, Moshe (2010). "Orbifolds as diffeologies" (PDF). Transactions of the American Mathematical Society. 362: 2811–2831. JSTOR 25677806.
10. ^ Gürer, Serap; Iglesias-Zemmour, Patrick (2019). "Differential forms on manifolds with boundary and corners". Indagationes Mathematicae. 30 (5): 920–929. doi:10.1016/j.indag.2019.07.004.
11. ^ Hain, Richard M. (1979). "A characterization of smooth functions defined on a Banach space". Proceedings of the American Mathematical Society. 77 (1): 63–67. doi:10.1090/S0002-9939-1979-0539632-8. ISSN 0002-9939.
12. ^ Losik, Mark (1992). "О многообразиях Фреше как диффеологических пространствах" [Fréchet manifolds as diffeological spaces]. Izv. Vyssh. Uchebn. Zaved. Mat. (in Russian). 5: 36–42 – via All-Russian Mathematical Portal.
13. ^ Losik, Mark (1994). "Categorical differential geometry". Cahiers de Topologie et Géométrie Différentielle Catégoriques. 35 (4): 274–290.
14. ^ Christensen, John Daniel; Sinnamon, Gordon; Wu, Enxin (2014-10-09). "The D -topology for diffeological spaces". Pacific Journal of Mathematics. 272 (1): 87–110. doi:10.2140/pjm.2014.272.87. ISSN 0030-8730.
15. ^ Donato, Paul; Iglésias, Patrick (1985). "Exemples de groupes difféologiques: flots irrationnels sur le tore" [Examples of diffeological groups: irrational flows on the torus]. C. R. Acad. Sci. Paris Sér. I Math. (in French). 301 (4): 127–130. MR 0799609.