Regularity of $R(X)$ does not pass to finite unions

J. F. Feinstein
2019 Proceedings of the American Mathematical Society  
We show that there are compact plane sets X, Y such that R(X) and R(Y) are regular but R(X ∪ Y) is not regular.
doi:10.1090/proc/14944 fatcat:kszn54wchfaifabzapzoyxps3i