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Mysterious "must have a value" error

asked 2021-01-20 11:51:15 +0200

Palomar gravatar image

I am using CoCalc to experiment with some GAP functionality in Sage. I am trying to run the following code.

G = gap.SmallGroup(64, 134)
R = gap.ResolutionPrimePowerGroupRadical(G, 10)

This yields the following error.

RuntimeError: Gap produced error output
Error, Variable: 'ResolutionPrimePowerGroupRadical' must have a value

What does this mean, and how do I solve this issue?

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What about G.ResolutionPrimePowerGroupRadical(10) ?

FrédéricC gravatar imageFrédéricC ( 2021-01-20 13:40:52 +0200 )edit

Apparently, this is not in GAP but in HAP ?

FrédéricC gravatar imageFrédéricC ( 2021-01-20 13:55:19 +0200 )edit

@FrédéricC It gives me the same error. And yes, as far as I understand this function comes from HAP but I don't really grasp what that means. I've tried to add the line gap.LoadPackage("hap") but it yields a similar "hap must have a value" error.

Palomar gravatar imagePalomar ( 2021-01-20 16:33:18 +0200 )edit

And in fact, not in HAP (that sage has) but in HAPprime (that sage does not have)

FrédéricC gravatar imageFrédéricC ( 2021-01-20 17:36:09 +0200 )edit

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answered 2021-01-21 18:33:57 +0200

slelievre gravatar image

The command you want requires the GAP package HAPprime, no longer being developed: its repository at GitHub is archived, and some of the code has been migrated to HAP.

That said, you can still install it by downloading the package's compressed tarball and unpacking it in your ~/.gap/pkg/ folder (after creating it if necessary).

To do that on CoCalc, open a CoCalc terminal and run the following commands (skip the initial $ on each line which represents the prompt):

$ GAP_PKG='~/.gap/pkg'
$ mkdir -p $GAP_PKG
$ cd $GAP_PKG
$ URL=
$ TGZ=happrime-0.6.tar.gz
$ wget $URL/$TGZ
$ tar xf $TGZ

Then you can use the desired GAP command, either directly in GAP or in Sage (via gap or, better, libgap) after loading the package HAPprime.

In Sage:

sage: libgap.LoadPackage("happrime")
#I  HAP warning: Set BROWSER_PATH manually if needed.
sage: G = libgap.SmallGroup(64, 134)
sage: R = libgap.ResolutionPrimePowerGroupRadical(G, 10)
sage: R
Resolution of length 10 in characteristic 2 for <pc group of size 64 with 6 generators> .
No contracting homotopy available.
A partial contracting homotopy is available.
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Asked: 2021-01-20 11:51:15 +0200

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Last updated: Jan 21