Cosine Sign Correlation (2212.02496v1)

Abstract: Fix $\left{a_1, \dots, a_n \right} \subset \mathbb{N}$, and let $x$ be a uniformly distributed random variable on $[0,2\pi]$. The probability $\mathbb{P}(a_1,\ldots,a_n)$ that $\cos(a_1 x), \dots, \cos(a_n x)$ are either all positive or all negative is non-zero since $\cos(a_i x) \sim 1$ for $x$ in a neighborhood of $0$. We are interested in how small this probability can be. Motivated by a problem in spectral theory, Goncalves, Oliveira e Silva, and Steinerberger proved that $\mathbb{P}(a_1,a_2) \geq 1/3$ with equality if and only if $\left{a_1, a_2 \right} = \gcd(a_1, a_2)\cdot \left{1, 3\right}$. We prove $\mathbb{P}(a_1,a_2,a_3)\geq 1/9$ with equality if and only if $\left{a_1, a_2, a_3 \right} = \gcd(a_1, a_2, a_3)\cdot \left{1, 3, 9\right}$. The pattern does not continue, as $\left{1,3,11,33\right}$ achieves a smaller value than $\left{1,3,9,27\right}$. We conjecture multiples of $\left{1,3,11,33\right}$ to be optimal for $n=4$, discuss implications for eigenfunctions of Schr\"odinger operators $-\Delta + V$, and give an interpretation of the problem in terms of the lonely runner problem.