exact_part for asymptotic expansions

[behackl]

For various reasons, having a method expansion.exact_part that returns the asymptotic expansion that consists of all ExactTerm-summands would be very useful.

This ticket provides such a method.

CC: @dkrenn @cheuberg

Component: asymptotic expansions

Author: Benjamin Hackl

Branch/Commit: 7276172

Reviewer: Clemens Heuberger

Issue created by migration from https://trac.sagemath.org/ticket/19931

[behackl]

Author: Benjamin Hackl

[behackl]

Commit: 8aa55e2

[behackl]

New commits:

8aa55e2

implement exact_part

[behackl]

Branch: u/behackl/asy/exact_part

[cheuberg]

comment:2

I'd prefer to copy the expression and to delete all non-exact elements. In general, this should be more efficient than recreating a poset with almost all elements --- after all, in general, there will be more exact terms than error terms.

[sagetrac-git]

Changed commit from 8aa55e2 to 7276172

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

7276172

delete terms from copy instead of building new poset

[behackl]

comment:4

Replying to @cheuberg:

I'd prefer to copy the expression and to delete all non-exact elements. In general, this should be more efficient than recreating a poset with almost all elements --- after all, in general, there will be more exact terms than error terms.

I agree, this strategy seems to be more efficient than building the entire expansion again. I've adapted the implementation.

[cheuberg]

Reviewer: Clemens Heuberger

[cheuberg]

comment:5

Doctests pass according to patchbot, documentation builds, code now fine.

[vbraun]

Changed branch from u/behackl/asy/exact_part to 7276172