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Comment by lxe | Hacker News Reader

I remember just playing with my good old TI-83 Plus Silver Edition, plugging a bunch of symbols together, like e, and i, etc... when I suddenly got the e^i*pi = -1.

How in the world do these completely unrelated (to me, at the time, at least) constants ended up with -1? It was mind blowing. This made me actually interested in math.

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If you realize where e comes from, which is compounding and what we call an exponential process, being that "the current rate of change is proportional to the present value", then the exponential of a complex number is no different to the exponent of a real number. In the case of one-dimensional real numbers you assign the slope of the function to the current value. The complex case is exactly the same, but if you think instead in the two-dimensional Argand plane and complex algebra, the slope is the tangent to a circle proportional to the angle at that radius (which curve is the one that at every point the slope is equal to its complex value?).

Therefore with the rules of complex arithmetic the tangent provides the "curling" effect in the curvature of a periodic circle, while in the real case you get the common compounding shape of the exponential curve. The relation to sine and cosine is just the projection into direct(real)/quadrature(imaginary) components in either fixed or intrinsic coordinates. Same when you expand the exponential into its complex power series.

BTW, this is actually the essence of infinitesimal transformations in continuous groups and the exponential map, which generalizes this concept to other types of numbers or abstract objects (i.e. Lie group theory).

I've been wondering about the connection between the "curling effect" and the exponential curve of e for a while. Do you have any links where I can learn more in depth?

With complex numbers in polar form the tangent is easy. In the unitary circle, if you derive e^(i*t) you get i*e^(i*t), which maps the cos(t) real component to i*cos(t) imaginary, and the i*sin(t) imaginary component to -sin(t) real. This is effectively a 90 degree rotation, so if you integrate the tangent infinitesimally over its path parameterized by t you will recover the circle.

Here is some introductory material to what I referenced above and some generalizations into more dimensions (which, as Hamilton discovered when stumbling into quaternions trying to augment complex numbers, is not as straightforward as you would think):

- Why i? [http://www.stat.physik.uni-potsdam.de/~pikovsky/teaching/stu...]

- An Introduction to Geometric Algebra with an Application in Rigid Body Mechanics [https://www.researchgate.net/profile/Terje-Vold/publication/...]

- Functions of Multivector Variables [https://journals.plos.org/plosone/article/file?id=10.1371/jo...]

- Lie Group Theory - A Completely Naive Introduction [https://jakobschwichtenberg.com/naive-introduction-lie-theor...]

- Previous HN discussion: Intuitive Understanding of Euler’s Formula [https://news.ycombinator.com/item?id=18325865]

Thanks to the way math is taught in India I still have very little understanding of so many of these concepts though I'm very good at remembering formulae/steps, mechanically calculating them, and applying them to real-world problems.

It wasn't until I stumbled upon Better Explained[1] I began to appreciate the need to understand math at a deeper/common-sensical level. Now I make it an effort to teach math to my son using examples, analogies so that he gets concepts like positional numbering system, why we need it, its advantages etc.,

https://betterexplained.com/articles/intuitive-understanding...

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