Arnold said the following in [a talk on teaching](http://pauli.uni-muenster.de/~munsteg/arnold.html):

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> Jacobi noted, as mathematics' most fascinating property, that in it one and the same function controls both the presentations of a whole number as a sum of four squares and the real movement of a pendulum.
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What's the meaning of this? Is the function he mentioned the theta function or something else? What's the relation with the real movement of a pendulum?