“…Theorem 1 for k = 2 was first established by the authors in [11] and for general k was first established by the second author in [49]. For k = 2, this result improves on work by Fogels [13], Roth [39], Richert [38], Rankin [37], Schmidt [41], Graham and Kolesnik [16], the second author [46,47], the first author [6], and the authors jointly [10]. For general k, this result improves on work by Halberstam and Roth [18], the first author [5], and the second author [48].…”