We consider the error term of the asymptotic formula for the number of pairs of k-free integers up to x. Our error term improves results by Heath-Brown, Brandes and Dietmann/Marmon. We then extend our results to r-tuples of k-free numbers and improve previous results by Tsang. Furthermore, we establish an error term for consecutive square-full integers. Finally, we will show that for all θ < 3 and for almost all D, the fundamental solution ǫ D associated to the Pell equation x 2 −Dy 2 = 1 satisfies ǫ D > D θ . This improves/recovers previous results by Fouvry and Jouve. The main tool of our work is the approximate determinant method.
AcknowledgmentI am very grateful to my supervisor Roger Heath-Brown who introduced me to the determinant method and its applications. I thank him for many constructive and helpful comments on my work. I would like to thank Étienne Fouvry and Florent Jouve for drawing my attention to their paper [5] which resulted in Theorem 5 below. I am also very grateful to the EPSRC 1 and to St. Anne's College, Oxford who are generously funding and supporting my doctorate's degree and this project.