Hooley's delta function

The sequence was first introduced by Paul Erdős in 1974,[1] then studied by Christopher Hooley in 1979.[2]

In 2023, Dimitris Koukoulopoulos and Terence Tao proved that the sum of the first ${\displaystyle n}$ terms, ${\displaystyle \textstyle \sum _{k=1}^{n}\Delta (k)\ll n(\log \log n)^{11/4}}$, for ${\displaystyle n\geq 100}$.[3] In particular, the average order of ${\displaystyle \Delta (n)}$ to ${\displaystyle k}$ is ${\displaystyle O((\log n)^{k})}$ for any ${\displaystyle k>0}$.[4]

Later in 2023 Kevin Ford, Koukoulopoulos, and Tao proved the lower bound ${\displaystyle \textstyle \sum _{k=1}^{n}\Delta (k)\gg n(\log \log n)^{1+\eta -\epsilon }}$, where ${\displaystyle \eta =0.3533227\ldots }$, fixed ${\displaystyle \epsilon }$, and ${\displaystyle n\geq 100}$.[5]

This function measures the tendency of divisors of a number to cluster.

The growth of this sequence is limited by ${\displaystyle \Delta (mn)\leq \Delta (n)d(m)}$ where ${\displaystyle d(n)}$ is the number of divisors of ${\displaystyle n}$.[6]

• Divisor function
• Euler's number