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Oct 18, 2018 at 15:03 comment added Peter Humphries @GHfromMO, yep, which is why it comes up in the field automorphic forms so often!
Oct 18, 2018 at 14:36 comment added GH from MO @PeterHumphries: Indeed the function looked familiar! It is also the index of $\Gamma_0(n)$ in $\Gamma_0(1)$ if I am not mistaken.
Oct 18, 2018 at 14:35 comment added GH from MO @MartinSleziak: Thanks for your comment. Clearly, one can also prove this statement by elementary means, by making use of the convolution I mentioned.
Oct 18, 2018 at 14:07 comment added Peter Humphries In the field of automorphic forms, this function is often denotes $\nu(n)$ (for example, in Iwaniec-Luo-Sarnak's seminal paper on low-lying zeroes of $L$-functions of holomorphic cuspidal eigenforms).
Oct 18, 2018 at 13:58 comment added Martin Sleziak I will just add this function appears in Exercise 11 in Chapter 3 in Apostol's book. The last part of this exercise is the asymptotic formula $\sum_{n\le x} \varphi_1(n)=\frac{\zeta(2)}{2\zeta(4)}x^2+O(x\log x)$. (Apostol denotes the function by $\varphi_1$.)
Oct 18, 2018 at 13:58 history edited GH from MO CC BY-SA 4.0
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Oct 18, 2018 at 13:53 vote accept Stanley Yao Xiao
Oct 18, 2018 at 13:50 history answered GH from MO CC BY-SA 4.0