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Sébastien Brisard commented on MATH863:

A good test would be {{FieldLUDecomposition}}. It might work, it might not. But even if it
works, it would not tell us much. Indeed, we would have to make sure that _all_ classes operating
on {{FieldElement}} take into account noncommutativity of the multiplication. There is no
way to check that!
I can give you a counter example (not included in CommonsMath) : I've developed a {{Polynomial<T
extends FieldElement<T>}} with coefficients as {{FieldElement}}s. Then I expand {{(a
+ b)^n}} using Pascal's rule [1]. While this is perfectly correct on a regular field, it would
certainly fail on a {{Quaternion}} (if I understand correctly). If {{Quaternion}} implemented
{{FieldElement}}, applying Pascal's rule would be allowed from the point of view of the compiler,
although pure nonsense from a mathematical point of view.
Therefore, I think {{Quaternion}} *should not* implement {{FieldElement}}. Did I miss something?
What do others think?
[1] {{(a + b)^2 = a^2 + a * b + b * a + b^2 != a^2 + 2 * a * b + b^2}} if {{*}} does not commute.
> new Quaternion class added in complex package
> 
>
> Key: MATH863
> URL: https://issues.apache.org/jira/browse/MATH863
> Project: Commons Math
> Issue Type: New Feature
> Affects Versions: 3.1
> Reporter: Julien Anxionnat
> Attachments: quaternion.patch, quaternion_v2.patch
>
> Original Estimate: 0h
> Remaining Estimate: 0h
>
> This patch provides a new class for the mathematical object "Quaternion" in the complex
package.
> This quaternion is considered as a mathematical object (the Hamilton's hypercomplex number).
> Note that it's not a rotation quaternion which has to be a quaternion of norm one. Although
this feature could be used for a getter in the Rotation class.
> This patch provides also some improvements in Precision class : a "double comparison
epsilon" and a method to compute relative comparison.
> (Please, note that's it's my first contribution, and I apologize in advance for my mistakes…)

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