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In devising challenging exercises for my students, I am tempted to give them something like $\cos(3\sin(4))$, but then I get to wondering whether such a calculation would ever be encountered in practice. Since radians are dimensionless, as are values returned by trig functions, there is no mathematical barrier to this happening, but I was wondering if it ever happened naturally in the course of solving some problem, in mathematics, physics, finance, or elsewhere.

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This might be of interest. –  J. M. Sep 4 '11 at 0:53
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up vote 15 down vote accepted

This does occur. A notable example would be the Bessel function $$J_n(x) = {1 \over \pi}\int_0^{\pi} \cos(nt - x\sin t)\,dt$$ These functions come up in various places in physics and so on. Also, whenever you do a contour integral such as ${\displaystyle \int_{|z| = 1}{1 \over \cos(z)}\,dz}$, if you parameterize the unit circle by $t \rightarrow e^{it}$ you will be doing the integral $$\int_0^{2\pi}{ie^{it} \over \sin(e^{it})}\,dt$$ Due to Euler's formula the denominator is effectively the composition of trigonometric functions. And contour integrals of such functions come up in applications all the time.

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Totally, totally cool! I love MSE! I have up-votd and accepted your answer. –  Mike Jones Sep 3 '11 at 21:38
    
thanks a lot :) –  Zarrax Sep 3 '11 at 21:41
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Tiny note: the first integral representation given coincides with a Bessel function only for integer $n$. For general $n$, the function is called an Anger function. There is also the related Weber function, where the $\cos$ is replaced by $\sin$. –  J. M. Sep 4 '11 at 0:21
    
@J.M.: Thanks. That's a great addendum. –  Mike Jones Sep 5 '11 at 19:35
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This occurs naturally when you express a plane wave $\mathrm e^{\mathrm i\mathbf k\mathbf x}$ in terms of the angle $\theta$ between $\mathbf k$ and $\mathbf x$ as $\mathrm e^{\mathrm ikx\cos\theta}=\cos(kx\cos\theta)+\mathrm i\sin(kx\cos\theta)$. This is especially relevant when expanding plane waves in terms of cylindrical or spherical waves, which is related to scattering and the Bessel functions mentioned by Zarrax.

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Thanks. This gets an up-vote from me. –  Mike Jones Sep 3 '11 at 22:50
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Phase modulation would come close.

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I took a glance at the link. It looks tangential, but I appreciate the info. –  Mike Jones Sep 3 '11 at 21:37
    
The connection is most clear if you view the signal as a sum of pure tones (via Fourier analysis). –  Henning Makholm Sep 3 '11 at 21:47
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