We prove the undecidability of MSO on $\omega$-words extended with the second-order predicate $U_1(X)$ which says that the distance between consecutive positions in a set $X \subseteq \mathbb{N}$ is unbounded. This is achieved by showing that adding $U_1$ to MSO gives a logic with the same expressive power as $MSO+U$, a logic on $\omega$-words with undecidable satisfiability. As a corollary, we prove that MSO on $\omega$-words becomes undecidable if allowing to quantify over sets of positions that are ultimately periodic, i.e., sets $X$ such that for some positive integer $p$, ultimately either both or none of positions $x$ and $x+p$ belong to $X$.

@article{BojanczykDGPS19,
  title = {Undecidability of a weak version of MSO+U},
  author = {Mikołaj Bojańczyk and Laure Daviaud and Bruno Guillon and Vincent Penelle and A. V. Sreejith},
  journal = {Logical Methods in Computer Science},
  volume = {Volume 16, Issue 1},
  year = {2020},
  doi = {10.23638/lmcs-16(1:12)2020}
}