At the end of page 5 of the Tao's lectures notes, he sets $\psi$ a Schwartz function supported on the unit cube $[0,1]^n$ and choose $f(x)=\sum_{k=1}^N\epsilon_k\psi(x-ke_1)$, where $e_1$ is one of the basis vectors of $\mathbb{R}^n$, $N$ is a large integer and $\epsilon_k$ are a collection of independent identically distributed signs $\epsilon_k=\pm1$. We have $\|f\|_p\sim N^{1/p}$ (here $A\sim B\Leftrightarrow A\lesssim B$ and $B\lesssim A$, where $A\lesssim B$ means that there is C such that $A\leq C.B$.) Using the Khinchin inequality: If $f_1,\ldots,f_N$ are a collection of functions and $\epsilon_k$ are randomized signs, then for any $1
I don't understand the following statement: there must exist some choice of signs for which $\|\hat{f}\|_q\gtrsim N^{1/2}$.
Help me, please?