Minimum of cosine sum
Suppose f(x) is the sum of terms of the form cos(kx) where k is an integer from a set A with n elements. Then the maximum value of f is f(0) = n. But what is the minimum value of f? The Chowla cosine conjecture says that the minimum should be less than −√n for large n. For now the best proven results are much smaller in absolute value [1]. I was playing around with this problem, and the first thing I thought of […]