Iwaniec Kowalski Notation

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On page 532 of the book analytic number theory by Iwaniec and Kowalski, the following notation is used:

$C^{~\infty}$ and $\tau(n,\chi)$.

Could anyone tell me what these represent? (the former is definitely not functions!)

Thanks!

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$c|C^{\infty}$ just means that $c|C^k$ for large $k$.

$\tau(n,\chi)$ is a twisted divisor function $\sum_{d|n} \chi(d)$.