Chromogeometry (2008)
arxiv.org
arxiv.org
The quadratic form in Blue is the norm squared of the complex numbers.
The quadratic form in Red is the norm squared of the split-complex numbers.
The quadratic form in Green is the norm squared of R⊕R, which is isomorphic to the split-complex numbers.
There's also the dual numbers. They're the only remaining 2d unitary algebra over the Real numbers, but they don't feature in the above paper.