Practice problem
Which of the following abelian groups have commutative endomorphism rings?
Q\mathbb{Q}Q
Z/2018\mathbb{Z}/2018Z/2018
Q2/Z2\mathbb{Q}_2/\mathbb{Z}_2Q2/Z2 (the Prüfer 222-group)
Z/2⊕Z/2\mathbb{Z}/2\oplus \mathbb{Z}/2Z/2⊕Z/2
Z/2⊕Z/3\mathbb{Z}/2\oplus \mathbb{Z}/3Z/2⊕Z/3
Type a concept, like “slope” or “photosynthesis”.