On the non-monotonicity of the denominator of generalized harmonic sums

Let ${\displaystyle \sum _{i=a}^b\frac{1}{i}=\frac{u_{a,b}}{v_{a,b}}}$ with $u_{a,b}$ and $v_{a,b}$ coprime. Erdős and Graham asked the following: Does there, for every fixed $a$, exist a $b$ such that $v_{a,b}<v_{a,b-1}$? If so, what is the least such $b=b(a)$? In this paper we will investigate these problems in a more general setting, answer the first question in the affirmative and obtain the bounds $a+0.54\log (a)<b(a)\le 4.374(a-1)$, which hold for all large enough $a$.