Is cosx+cos(sqrt(2)x) periodic?

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SUMMARY

The function cos(x) + cos(sqrt(2)x) is not periodic. This conclusion is reached by analyzing the periodicity conditions, specifically that for the function to be periodic, there must exist a common period T that satisfies both cos(x) and cos(sqrt(2)x). Since the periods of these functions do not align, there is no integer T that can satisfy the periodicity condition. The discussion highlights that while functions like cos(2x) and cos(3x) are periodic due to their rational relationships, the irrational coefficient in sqrt(2) disrupts this property.

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Homework Statement



Is the function cosx + cos(sqrt(2)x) is periodic?

Homework Equations



cos(x)=cos(x+2pi)

The Attempt at a Solution



For the above function to be periodic:
cosx + cos(sqrt(2)x) = cos(x+T) + cos(sqrt(2)(x + T))
Does that imply that 2pi = T AND 2pi = sqrt(2)T, ergo there is no such T?
 
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It doesn't exactly imply that. However, by the definition of periodicity, you need integer T to satisfy the equation. For example, the function cos(2x)+cos(3x) is periodic, one has period \pi and the other has period 2\pi/3. It is not hard to see that both of these functions are also periodic by 2\pi, and hence their sum is also periodic. Returning to your question, your answer is correct, the function is not periodic. However, your equations seem a bit fallacious to me. All you need is that the period of the sum is the least common multiple of the periods of the summands.

Can you see such a least common multiple?
 

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