Coefficients for odd order about findiff

[maroba]

You are right. It should rather say "any even accuracy order". In case of requested odd accuracy orders, I chose to automatically fall back to the next higher even accuracy order, because - being more accurate - this should do no harm. However, I see your point and it may be cleaner to raise an exception instead.

Regarding your question on why only even orders: When I originally wrote the package, I couldn't think of a use case where one would need to use asymmetric coefficients in the interior of the grid instead of symmetric ones. And for symmetric coefficients, odd accuracy orders do not exist. Since one wouldn't want to mix different accuracy orders on the same grid, I saw no need for odd orders.

[stephenbeckr]

I see. Yes, I think an exception probably makes more sense.

A simple use case for asymmetric odd-order are the backward differentiation formulas (https://en.wikipedia.org/wiki/Backward_differentiation_formula). Of course these only need to be derived once, and we don't use arbitrarily high-order BDF since they're unstable.