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LMFDB / lmfdb

L-Functions and Modular Forms Database
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Wrong homogenization for j-map #5589

Open jvoight opened 1 year ago

jvoight commented 1 year ago

For https://beta.lmfdb.org/ModularCurve/Q/3.24.0-3.a.1.1/ there is an x in both numerator and denominator.

(@roed314 says we should move bug reports here, now that modular curves are in beta?)

jvoight commented 1 year ago

Maybe this is just an issue that it is homogenized using the (SL_2-)index, when it should be homogenized using the PSL_2-index?

I'm all for getting stacky, but we say the map is degree 12, and writing it as a rational function means we should be thinking on the coarse space.

koffie commented 9 months ago

Note that https://beta.lmfdb.org/ModularCurve/Q/3.12.0.a.1/ has the same issue which has SL_2 index = PSL_2 index. Although that might be due to the j map only being computed once and then used for both of these curves.

roed314 commented 8 months ago

This is fixed by this commit in the data generation code, so when we regenerate data it will get better.