Runs of integers with constant values of the Carmichael function

Abstract

In 2023, the first author and Vandehey proved that the largest $k$ for which the string of equalities $λ(n+1)=λ(n+2)=\cdots=λ(n+k)$ holds for some $n\leq x$, where $λ$ is the Carmichael $λ$ function, is bounded above by $O\left((\log x\log\log x)^2\right)$. Their method involved bounding the value of $λ(n + i)$ from below using the prime factorization of $n + i$ for each $i \leq k$. They then used the fact that every $λ(n + i)$ had to satisfy this bound. Here we improve their result by incorporating a reverse counting argument on a result of Baker and Harman on the largest prime factor of a shifted prime.