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"fraction" fixes - #4462

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pzinn wants to merge 6 commits into
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pzinn:frac
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"fraction" fixes#4462
pzinn wants to merge 6 commits into
Macaulay2:developmentfrom
pzinn:frac

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@pzinn

@pzinn pzinn commented Jul 2, 2026

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This fixes two somewhat related bugs in the handling of fractions:

R=QQ[a,b]/(a*b-1)
1/a -- before: fails to realise it's b, after: gives b
lift(oo,R) -- before: fails to lift, after: lifts

basically, M2 did not correctly simplify the denominator when it was a unit, leading to lots of oddities (this is only the simplest example). now it does a much cleaner job.

AI disclosure

I formulated the fixes, but codex helped with the c++.

@pzinn pzinn changed the title "frac" fixes "fraction" fixes Jul 2, 2026
@d-torrance d-torrance added the Engine Macaulay2/e label Jul 2, 2026
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d-torrance requested changes Jul 18, 2026
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Reviewed with @local-ring. Just a couple questions and comments

Comment thread M2/Macaulay2/e/rings/frac.cpp
Comment thread M2/Macaulay2/e/rings/frac.cpp Outdated
Comment thread M2/Macaulay2/tests/normal/frac.m2
@d-torrance d-torrance linked an issue Jul 19, 2026 that may be closed by this pull request
error multiplying matrices with entries in fraction field #4461
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d-torrance
d-torrance requested changes Aug 6, 2026
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Thanks for making the changes! I have a few other suggestions. (Thanks to Claude 🤖 for suggesting a few things, too.)

Comment thread M2/Macaulay2/packages/LocalRings/tests.m2 Outdated
Comment thread M2/Macaulay2/e/rings/frac.cpp
Comment thread M2/Macaulay2/e/rings/frac.cpp Outdated
Comment thread M2/Macaulay2/e/rings/frac.cpp Outdated
Comment thread M2/Macaulay2/e/rings/frac.cpp Outdated
@pzinn

pzinn commented Aug 9, 2026 •
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all suggestions implemented. these are minimal fixes, e.g., for the example of #3973, M2 will output an error message instead of crashing; I have not tried to make it produce the actual inverse.

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@d-torrance d-torrance d-torrance requested changes
@local-ring local-ring Awaiting requested review from local-ring

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error multiplying matrices with entries in fraction field

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