annoyance with res

[David Eisenbud]

Is anyone else annoyed with having to type 

 LengthLimit => 

every time you compute a resolution over a factor ring of a polynomial ring? The following fixes the problem when executed within an M2 session, but causes a bug when it's in my init.m2 file. How can I fix this?

Res = method(Options => options res)
Res Ideal := o -> I -> if o.LengthLimit == infinity then 

    res(I, LengthLimit => numgens ring I) else res I
Res Module := o -> M -> if o.LengthLimit == infinity then 

    res(M, LengthLimit => numgens ring I) else res M

--

David Eisenbud

Professor of Mathematics

University of California Berkeley

https://eisenbud.github.io/

[Doug Torrance]

Probably init.m2 is being loaded before Complexes (or OldChainComplexes, whichever you prefer), which defines res. So you could add an additional line right before:

needsPackage HomologicalAlgebraPackage

Res = ...

[David Eisenbud]

That fixes it -- thanks!

David

--

David Eisenbud

Professor of Mathematics

University of California Berkeley

https://eisenbud.github.io/

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[Mahrud Sayrafi]

My suggestion in #3166 was to make this essentially the default behavior, so that passing LengthLimit => infinity doesn't automatically produce an error (e.g. maybe compute up to numgens R + 1, as Devlin suggested in #3656, and if the last term is nonzero, print the first numgens R terms and change the printed result to explicitly indicate that the resolution continues). Locally, I've made a change similar to yours to avoid the headache, but it's very inconvenient when I switch to a server or send my scripts to someone else.

I would really prefer a global solution before Complexes becomes the default.

Mahrud

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[mike stillman]

Would you be satisfied with a different solution?

Specifically, would the following functionality meet your needs?

res(Ideal, ZZ) would be the same as res(Ideal, LengthLimit => ZZ)? (Same with Module as well).

so one would do 

`res(I, 5) `

to get lengthlimit of 5.

-- Mike

On Sunday, May 3, 2026 at 1:23:46 PM UTC-4 d...@berkeley.edu wrote:

[David Eisenbud]

My feeling is that Mahrud's solution (including the output that the resolution continues beyond the number of variables!) is the best; mine is second-best; and Mikes is an improvement over status quo, maybe sharing 2nd place...

David

--

David Eisenbud

Professor of Mathematics

University of California Berkeley

https://eisenbud.github.io/

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