Moment conjecture for Steinhaus random multiplicative functions

Let ff be a Steinhaus random multiplicative function: the values f(p)f(p) are independent and uniformly distributed on the complex unit circle, and f(n)=pαnf(p)αf(n)=\prod_{p^{\alpha}\parallel n}f(p)^\alpha. For q0q\geq0, define the random sum SN=nNf(n)S_N=\sum_{n\leq N}f(n) and let C(q)C(q) denote the conjectural leading constant.

Random-multiplicative-function moment conjecture. As NN\rightarrow\infty,

ESN2q{C(q)Nq,0q1,C(q)Nq(logN)(q1)2,1q.\mathbb{E}|S_N|^{2q}\sim \begin{cases} C(q)N^q,&0\leq q\leq1,\\ C(q)N^q(\log N)^{(q-1)^2},&1\leq q. \end{cases}

This is presented as a counter-conjecture to Helson's first-moment claim: at q=1/2q=1/2, it predicts order N\sqrt N rather than o(N)o(\sqrt N). It is motivated by the paper's moment asymptotics, while the full piecewise prediction remains conjectural.

Sources & referencesView supporting material

Primary source

Adam J. Harper, Ashkan Nikeghbali and Maksym Radziwiłł, “A note on Helson's conjecture on moments of random multiplicative functions”, arXiv:1505.01443 (2015).

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