Published online by Cambridge University Press: 20 November 2018
The function $\Delta (x,N)$ as defined in the title is closely associated via $\Delta \,(N)\,=\,{{\sup }_{x}}\,|\,\Delta (x,N)|$ to several problems in the upper bound sieve. It is also known via a classical theorem of Franel that certain conjectured bounds involving averages of $\Delta (x,N)$ are equivalent to the Riemann Hypothesis. We improve the unconditional bounds which have been hitherto obtained for $\Delta (N)$ and show that these are close to being optimal. Several auxiliary results relating $\Delta (Np)$ to $\Delta (N)$, where $p$ is a prime with $p\nmid N$, are also obtained and two new conjectures stated.