bug in revert method for lazy power series #35261
Description
Is there an existing issue for this?
- I have searched the existing issues for a bug report that matches the one I want to file, without success.
Did you read the documentation and troubleshoot guide?
- I have read the documentation and troubleshoot guide
Environment
- **OS**:
- **Sage Version**: 10.0.beta3Steps To Reproduce
No response
Expected Behavior
sage: L = LazyPowerSeriesRing(QQ, 'z')
sage: f = lambda n: 1 if ZZ(n).is_power_of(2) else 0
sage: lazy_f = L(f); lazy_f
z + z^2 + z^4 + O(z^7)
sage: lazy_f.revert()
z - z^2 + 2*z^3 - 6*z^4 + 20*z^5 - 70*z^6 + 256*z^7 + O(z^8)
Actual Behavior
sage: L = LazyPowerSeriesRing(QQ, 'z')
sage: f = lambda n: 1 if ZZ(n).is_power_of(2) else 0
sage: lazy_f = L(f); lazy_f
z + z^2 + z^4 + O(z^7)
sage: lazy_f.revert()
<repr(<sage.rings.lazy_series_ring.LazyPowerSeriesRing_with_category.element_class at 0x7f3209489720>) failed: ValueError: inverse does not exist>
Additional Information
Strangely, when we ask for any slice of the coefficients, then method starts to work:
sage: L = LazyPowerSeriesRing(QQ, 'z')
sage: f = lambda n: 1 if ZZ(n).is_power_of(2) else 0
sage: lazy_f = L(f); lazy_f
z + z^2 + z^4 + O(z^7)
sage: lazy_f.revert()
<repr(<sage.rings.lazy_series_ring.LazyPowerSeriesRing_with_category.element_class at 0x7f3209489720>) failed: ValueError: inverse does not exist>
sage: lazy_f[:1]
[]
sage: lazy_f.revert()
z - z^2 + 2*z^3 - 6*z^4 + 20*z^5 - 70*z^6 + 256*z^7 + O(z^8)