At the start of my first year of university in a Foundations module, my lecturer stated Euler's formula for complex numbers, eiθ=cosθ+isinθ. In passing he mentioned this is just a definition which conveniently works when you plug the numbers into the Taylor expansions and that you could actually define it differently (a number other than e). What would be the consequences of this? Is there any other number which gives interesting results?
Answer
Suppose you want the following desirable properties:
exp:θ↦eiθ is smooth
exp(x+y)=exp(x)⋅exp(y)
the first requirement is reasonable since we want the function to be nice, and the second is desirable, because it says most essentially that if we add angles,
we get
(cos(x)+isin(x))(cos(y)+isin(y))
which after working out with some trigonometric identities reduces to
cos(x+y)+sin(x+iy)
so that adding angles does what we would expect on the unit circle.
For any function with these two properties: f(0)=1 and differentiating with respect to x and letting x=0 gives
f′(y)=f(y)⋅f′(0)
which solving returns an exponential function Ceaθ where a=f′(0), although the requirement that f(0)=1 forces that C=1.
In this case, one could in principle define exponential functions with different bases, but this seems to me inconvenient.
The special thing about base e is that the derivative at 0 is precisely 1, which is why we get the clean formulas.
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