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Let $n_1 < \dots < n_N$ be positive integers. Assume we don't know anything about their actual values. What is the best general upper bound we can give for $$ \mu \left( x \in [0,1] : ~\left|\sum_{k=1}^N e^{2 \pi i n_k x} \right| > \kappa \sqrt{N} \right)? $$ Here $\mu$ is Lebesgue measure on $[0,1]$, and $\kappa$ is a parameter (imagine $\kappa$ to be somewhere around $\sqrt{\log N}$).

Of course we can use $\|\sum_{k=1}^N e^{2 \pi i n_k x}\|_2 = \sqrt{N}$ plus Chebyshev's inequality, to get $\mu \leq \kappa^{-2}$. But is there anything smarter we can do? Taking other norms than $L^2$-norm does not work, since in general these norms can get very large and the result will be worse than using $L^2$ norm. For example when $n_1 , \dots, n_N = 1, \dots, N$, we have an $L^4$ norm as large as $N^{3/4}$ - however, the actual size of the exceptional set is very small, so arguing with $L^4$ norms would be stupid in that situation. So is there any general estimate which beats the $L^2$ estimate?

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    $\begingroup$ You should be able to do better using some version of the Azuma-Hoeffding inequality? Or Bernstein's inequality? $\endgroup$ – pseudocydonia Oct 10 '19 at 17:16
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    $\begingroup$ No, this is not applicable here, since the trigonometric polynomial is not a sum of independent random variables. There is an analogue of Bernstein's inequality, but only if the sequence $(n_k)_k$ grows very quickly, e.g. exponentially. See for example Kac, M.: On the distribution of values of sums of the type $\sum f(2^k t).$ Ann. of Math. (2) 47 (1946), 33–49. $\endgroup$ – Kurisuto Asutora Oct 11 '19 at 7:53
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Your example $n_i=i$ shows that the $L^2$ bound is sharp when $\kappa \sim \sqrt{N}$ (though of course for $\kappa > \sqrt{N}$ the LHS vanishes). For smaller values of $\kappa$ one can simply combine this construction with a generic trigonometric series. In particular, if we have $N = M L$ for some integers $M,L$ with $M$ comparable to a large multiple of $\kappa^2$, we can consider the trigonometric polynomial $$ (\sum_{m=1}^M e^{2\pi i m x}) (\sum_{l=1}^L e^{2\pi i a_l x})$$ where the $a_l$ are some very widely separated frequencies in general position (e.g. $a_l = A^l$ for some large $A$). This is a polynomial of the required form $\sum_{k=1}^N e^{2\pi i n_k x}$ if the $a_l$ are sufficiently widely separated. Informally, the first factor $\sum_{m=1}^M e^{2\pi i m x}$ is of size $\sim M$ when $x = O(1/M)$, and the second factor is typically of size $\sqrt{L}$, so one expects the product to be of size $\sim M \sqrt{L} \geq \kappa \sqrt{N}$ on a set of size $\sim 1/M \sim \kappa^{-2}$, thus demonstrating the sharpness of the $L^2$ bound. One can make these calculations more rigorous by computing the moments $$ \int_0^1 |\sum_{m=1}^M e^{2\pi i m x}|^{2+2j} |\sum_{l=1}^L e^{2\pi i a_l x}|^{2j}\ dx$$ for $j=1,2$, and using the Paley-Zygmund inequality (with respect to the probability measure coming from normalising $|\sum_{m=1}^M e^{2\pi i m x}|^2\ dx$); I'll leave this as an exercise to the interested reader.

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  • $\begingroup$ Thank you, this is very helpful. If I understand correctly, then in probabilistic terms the polynomial here is constructed as a sum of (essentially) independent objects, with the independence coming from the separation of frequencies. Do you have any intuition if a similar example could also be constructed without resorting to this sort of independence? More specifically, is the $L^2$ bound still sharp if we additionally could assume that the $n_1, \dots, n_N$ are of comparable size, such as $n_1, \dots, n_N \in [R,2R]$ for some $R$? $\endgroup$ – Kurisuto Asutora Oct 11 '19 at 7:52
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    $\begingroup$ If one shifts all the $n_i$ in my example by a large factor $R$ then they will all lie in $[R,2R]$ but the distribution of the magnitude of the trignometric sum is unchanged. So if $R$ is allowed to be really large (E.g. exponentially large in $N$) this doesn't help at all. However one might be able to do something in the opposite regime when $R$ is comparable to $N$ and there is not enough "room" to have independent behaviour. $\endgroup$ – Terry Tao Oct 11 '19 at 16:29
  • $\begingroup$ It should be noted that one also wants the $a_l$'s large so that the second factor is indeed typically of size $\sqrt{L}$ (obviously for $x$ close to $0$, the second factor is of size $L$ -- the point is the "close to" depends on the size of the $a_l$'s). $\endgroup$ – mathworker21 Oct 14 '19 at 15:16

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