Induced/coinduced diffeologies and inductions/subductions #
In large parts based on Mathlib.Topology.Order
.
The coarsest diffeology on X
such that f : X → Y
is smooth, also known as the
pullback diffeology.
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The finest diffeology on Y
such that f : X → Y
is smooth, also known as the
pushforward diffeology.
While this is just the infimum of all diffeologies that make f
smooth, we use a slightly
more involved construction to make the D-topology defeq to the coinduced topology.
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- DiffeologicalSpace.coinduced f dX = (sInf {d : DiffeologicalSpace Y | DSmooth f}).withDTopology (TopologicalSpace.coinduced f DTop) ⋯
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The coinduced diffeology is the infimum of all diffeologies that make f
smooth.
The diffeology coinduced by f : X → Y
is generated by all plots of the form f ∘ p
for plots p
in X
.
The D-topology of the coinduced diffeology agrees with the coinduced topology.
The D-topology of the induced diffeology is always at least as fine as the induced topology.
For functions whose range is D-open, the D-topology of the induced diffeology agrees with the induced topology.
We call a map "D-inducing" (short for diffeologically inducing) if its domain carries the diffeology induced by it. This doesn't seem to appear in the literature, but we introduce it as a weaker version of inductions for situations where injectivity isn't actually needed.
This is analogous to inducing maps in topology, whereas inductions are analagous to embeddings.
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An induction is a map between diffeological spaces that is both inducing and injective. This is analogous to embeddings in topology.
- injective : Function.Injective f
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We call a map "D-coinducing" (short for diffeologically coinducing) if its codomain carries the diffeology coinduced by it. This doesn't seem to appear in the literature, but we introduce it as a weaker version of subductions for situations where surjectivity isn't actually needed.
This is analogous to coinducing maps in topology, whereas subductions are analagous to quotient maps.
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A subduction is a map between diffeological spaces that is both coinducing and surjective. This is analogous to quotient maps in topology.
- surjective : Function.Surjective f
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An open induction is an induction that is also an open map with respect to the D-topologies. What makes this an especially interesting notion compared to e.g. closed inductions is that while inductions are not always embeddings, open inductions are always open embeddings; furthermore, inductions are open iff their range is open. In comparison, an induction whose range is closed can still be both not an embedding and not a closed map.
- isOpenMap : IsOpenMap f