bug in composition of power series

[aghitza]

The following returns an incorrect answer:

sage: S.<z> = QQ[[]]
sage: p = 1 + O(z)
sage: q = 1 + z
sage: p(q)          # should return O(z^0)
1

This was reported via "report a problem" from the notebook.

Component: algebra

Keywords: power series composition

Reviewer: David Loeffler

Issue created by migration from https://trac.sagemath.org/ticket/5367

[aghitza]

comment:1

As pointed out by Ralf Hemmecke on sage-devel, the correct behaviour is not to return 1, but rather to raise an error if q has a constant term.

[loefflerd]

comment:2

I just came across this, which is presumably related:

sage: R.<x> = QQ[[]]
sage: f = 1 + 24*x^11 + 24*x^22 + O(x^33)
sage: f(x^2)
1 + 24*x^22 + 24*x^44 + O(x^86)

The answer should clearly be 1 + 24x^22 + 24x^44 + O(x^66). (This is causing some headaches in trying to sort out degeneracy maps for modular forms.)

[robertwb]

comment:3

See also #3979.

[JohnCremona]

comment:4

Since this is a duplicate of #3979, I think this ticket should be closed.

[sagetrac-fwclarke]

comment:5

Replying to @robertwb:

See also #3979.

Which now has a patch.

[loefflerd]

comment:6

I concur that this should be closed as duplicate. I'll set it to "positive review" to bring this to the attention of the release manager.

[jdemeyer]

Reviewer: David Loeffler