genus 2 curve with a Jacobian of dimension 4?

[AndrewVSutherland]

This looks like a bug to me (but possibly I am misunderstanding the output):

> C:=HyperellipticCurve(x^3 + x^2,-x^5 - x^4 + 2*x^2 + 2*x + 9);
> HeuristicEndomorphismAlgebra(C);
<[ <1, 1> ], [
<1, 1, [ -1, 1 ], 1, 4>
], <1, -1>, 1>
Associative Algebra of dimension 1 with base ring Rational Field
[ (1) ]
> HeuristicSplitting(C);
[
[ 4, 1 ]
]

This is on Magma V2.28-9

[JRSijsling]

The arguments should be interchanged. That said, this is not the first time that this happens, which means that the function should just accept them in the order that you suggest.

[AndrewVSutherland]

Sorry, I'm being an idiot, I meant to cut to do

> C:=HyperellipticCurve(-x^5 - x^4 + 2*x^2 + 2*x + 9,x^3 + x^2);                                                     
> HeuristicSplitting(C);
> 
> 1+1;
2
> C;
Hyperelliptic Curve defined by y^2 + (x^3 + x^2)*y = -x^5 - x^4 + 2*x^2 + 2*x + 9 over Rational Field
> Genus(C);
2
> HeuristicSplitting(C);

In file "/home/drew/dev/magma-2.28-8/package/Geometry/RieSrf/fundamentalgroup.m", line 609, column 22:
>>         if E1`r le E2`r then
                        ^
Runtime error in `: Attribute 'r' for this structure is valid but not assigned

> HeuristicDecomposition(C);

In file "/home/drew/dev/magma-2.28-8/package/Geometry/RieSrf/fundamentalgroup.m", line 609, column 22:
>>         if E1`r le E2`r then
                        ^
Runtime error in `: Attribute 'r' for this structure is valid but not assigned

But I'll create a new issue for this