Real zeros of random trigonometric polynomials with -periodic coefficients
Volume 174 / 2023
Abstract
The large degree asymptotic of the expected number of real zeros of a random trigonometric polynomial T_n(x) = \sum_{j=0} ^{n} (a_j \cos (j x) + b_j \sin (j x)), \quad x \in (0,2\pi ), with i.i.d. real-valued standard Gaussian coefficients is known to be 2n / \sqrt {3} . In this article, we consider quite a different and extreme setting on the set of coefficients of T_n . We show that a random trigonometric polynomial of degree n with \ell -periodic i.i.d. Gaussian coefficients is expected to have significantly more real zeros compared to the classical case with i.i.d. Gaussian coefficients. More precisely, the expected number of real zeros of T_n is proportional to the degree n with a proportionality constant \mathrm C_{\ell ,r} \in (\sqrt{2},2] where \ell is the period of the coefficients, and r is the remainder of the Euclidean division of n by \ell . This constant is explicitly represented by a double integral formula. The case r=0 is marked as a special one since T_n in such a case asymptotically obtains the largest possible number of real zeros.