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Conversion from formal series to power/LaurentSeriesRing silently truncate negative term #39839

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user202729 opened this issue Mar 31, 2025 · 1 comment · May be fixed by #39841
Open
2 tasks done

Conversion from formal series to power/LaurentSeriesRing silently truncate negative term #39839

user202729 opened this issue Mar 31, 2025 · 1 comment · May be fixed by #39841
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t: bug

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@user202729
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user202729 commented Mar 31, 2025 •
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Steps To Reproduce

sage: var("x")
....: f = (1/x+sqrt(x+1)).series(x)
x
sage: f
1*x^(-1) + 1 + 1/2*x + (-1/8)*x^2 + 1/16*x^3 + (-5/128)*x^4 + 7/256*x^5 + (-21/1024)*x^6 + 33/2048*x^7 + (-429/32768)*x^8 + 715/65536*x^9 + (-2431/262144)*x^10 + 4199/524288*x^11 + (-29393/4194304)*x^12 + 52003/8388608*x^13 + (-185725/33554432)*x^14 + 334305/67108864*x^15 + (-9694845/2147483648)*x^16 + 17678835/4294967296*x^17 + (-64822395/17179869184)*x^18 + 119409675/34359738368*x^19 + Order(x^20)
sage: PowerSeriesRing(QQ, "x")(f)
1 + 1/2*x - 1/8*x^2 + 1/16*x^3 - 5/128*x^4 + 7/256*x^5 - 21/1024*x^6 + 33/2048*x^7 - 429/32768*x^8 + 715/65536*x^9 - 2431/262144*x^10 + 4199/524288*x^11 - 29393/4194304*x^12 + 52003/8388608*x^13 - 185725/33554432*x^14 + 334305/67108864*x^15 - 9694845/2147483648*x^16 + 17678835/4294967296*x^17 - 64822395/17179869184*x^18 + 119409675/34359738368*x^19 + O(x^20)
sage: LaurentSeriesRing(QQ, "x")(f)
1 + 1/2*x - 1/8*x^2 + 1/16*x^3 - 5/128*x^4 + 7/256*x^5 - 21/1024*x^6 + 33/2048*x^7 - 429/32768*x^8 + 715/65536*x^9 - 2431/262144*x^10 + 4199/524288*x^11 - 29393/4194304*x^12 + 52003/8388608*x^13 - 185725/33554432*x^14 + 334305/67108864*x^15 - 9694845/2147483648*x^16 + 17678835/4294967296*x^17 - 64822395/17179869184*x^18 + 119409675/34359738368*x^19 + O(x^20)

Expected Behavior

The conversion to PowerSeriesRing ought to raise an error, be and the conversion to LaurentSeriesRing ought to keep the 1*x^(-1) term.

Checklist

  • I have searched the existing issues for a bug report that matches the one I want to file, without success.
  • I have read the documentation and troubleshoot guide
This was referenced Mar 31, 2025
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@user202729
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The linked pull request needs more work (most of it is to modify the doctests in LazySeries that raise error in #39841, and a bit of it is fix the zero-padding behavior)

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Fix conversion from SymbolicSeries to LaurentSeries user202729/sage
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