Evaluating $\lim_{x\to \infty} \cos^{x^{2}}\left(\frac{a}{x}\right)$

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I have the following limit:

$$\lim_{x\to \infty} \cos^{x^{2}}\left(\dfrac{a}{x}\right)$$

where $a$ can be any number.

How can I calculate it?

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$$\log\left(\cos^{x^2}(a/x)\right)= x^2\log\cos(a/x)=x^2\log\left(1-\frac {a^2}{2x^2}+O(x^{-4})\right) =x^2\left(-\frac {a^2}{2x^2}+O(x^{-4})\right)$$ so that $\cos^{x^2}(a/x)\to \exp(-a^2/2)$ as $x\to\infty$.