Faulted demonstration of orthonormality in my textbook?

LeFH18
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Homework Statement
Show that the functions:

ψ_m(θ)= (2π)^(-1/2) e(imθ), m = 0, +- 1, +-2, ...

form an orthonormal set over the interval (0,2π)
Relevant Equations
Function set:
ψ_m(θ)= (2π)^(-1/2) e(imθ), m = 0, +- 1, +-2, ...

Orthonormality condition:

int( ψ*_m ψ_n dx) = δ_nm

where δ is Kronecker's delta
The textbook, "Mathematics for Physical Chemistry - Opening doors" (McQuarrie), solves this example excercise as follows:
1738621436173.png

1738621466635.png


And is the case for n =/= m which I'm troubled with, because, if I solve the integral instead of using the cycles argument, I get that:
1738621585832.png


And I can't see how this is equal to zero. Any leads on why this must be zero for n unequal of m?
 
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Euler's identity says ##e^{i \varphi}=\cos \varphi + i \sin \varphi.## Now substitute ##\varphi = 2\pi (n-m) .## Orthogonality of two vectors ##v_m, v_n## means ##\bigl\langle v_m,v_n \bigr\rangle =\delta_{mn}.## It's the definition, whether ##v## are vectors or functions, and whatever the inner product is defined as, in this case by an integral.
 
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Thank you so much!
I did not realize that I could do that substitution. Doing that, I get the following:
## = \frac{i}{\varphi}(1-e^{\varphi i})##
Expanding it into cosine and sine, and noting that ##(n-m)## is always an integer because ##n## and ##m## also are integers, it leads to the inner product being zero.
 
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...

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