Implement characteristic() for ring of coordinate functions

[jdemeyer]

The characteristic should be the characteristic of the K from this documentation:

r"""
Symbolic Coordinate Functions

In the context of a topological manifold `M` over a topological field `K`,
a *coordinate function*  is a function from a chart codomain
to `K`. In other words, a coordinate function is a `K`-valued function of
the coordinates associated to some chart.

Now please tell me: how does one retrieve the K from the coordinate ring?

On the other hand, the focus on symbolics might imply that the characteristic is always zero, but it would be good if somebody could confirm this.

[jdemeyer]

Thinking more about it, this could be handled in the category of Algebras.

[jdemeyer]

Replying to jdemeyer:

Thinking more about it, this could be handled in the category of Algebras.

Not really, since you cannot in general know the base ring of an element of Algebras.

[chapoton]

New commits:

​86d0ef3

trac 23329 characteristic of coordinate chart function ring

[chapoton]

green bot, please review

[roed]

Looks good to me.

[chapoton]

so is this a positive review ?

[roed]

Oops, forgot to modify the ticket.