Woronowicz, Mateusz. “A Note on the Square Subgroups of Decomposable Torsion-Free Abelian Groups of Rank Three”. Annales Mathematicae Silesianae, vol. 32, Oct. 2017, pp. 319-31, https://trrest.vot.pl/ojsus/index.php/AMSIL/article/view/13930.