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Monday, April 29, 2019

Representing a complex line as a directed ellipse

Consider nonzero v = v_r + iv_i \in \mathbb{C}^n, It can be thought of as an ordered 2-tuple of vectors (v_r, v_i)\in \mathbb{R}^n\times\mathbb{R}^n.

The complex line generated by v is
\{r[(\cos(\theta) v_r -\sin(\theta) v_i) + i(\sin(\theta) v_r +\cos(\theta) v_i)]:\\ r\ge 0, \theta\in[0, 2\pi]\}

So, essentially, it is a set of concentric ellipses. We consider one of them:
\{(\cos(\theta) v_r -\sin(\theta) v_i) + i(\sin(\theta) v_r +\cos(\theta) v_i): \theta\in[0, 2\pi]\}

As \theta increases, both (\cos(\theta) v_r -\sin(\theta) v_i) and (\sin(\theta) v_r +\cos(\theta) v_i) rotate in the ellipse, always being conjugate to each other.

So we could represent the complex line generated by v as a directed ellipse, with the direction determined by v_r "pointing counterclockwise towards" v_i.

Multiplying v by e^{i\theta}, then, is to rotate the conjugate pair on the ellipse clockwise by \theta.

Conjugating the complex line is then reversing the direction of the ellipse. Conjugation does not change the line iff the ellipse is degenerate, that is, v_r, v_i are \mathbb{R}-linearly dependent.

I found this perspective very new and helpful, but I have never seen this presented this way before. I asked on math.stackexchange but nobody replied. Would anyone point me to a reference?

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