Corner, A. L. S., Groups of units of orders in Q-algebras, Models, modules and abelian groups, Walter de Gruyter, Berlin: 9–61, 2008, MR 2513226, doi:10.1515/9783110203035.9. See in particular the proof of Lemma H.4, p. 36, which uses this fact.
Blass, Andreas, Injectivity, projectivity, and the axiom of choice, Transactions of the American Mathematical Society, 1979, 255: 31–59, JSTOR 1998165, MR 0542870, doi:10.1090/S0002-9947-1979-0542870-6. For the connection to free objects, see Corollary 1.2. Example 7.1 provides a model of set theory without choice, and a non-free projective abelian group in this model that is a subgroup of a free abelian group , where is a set of atoms and is a finite integer. Blass writes that this model makes the use of choice essential in proving that every projective group is free; by the same reasoning it also shows that choice is essential in proving that subgroups of free groups are free.
Johnson, D. L., Topics in the Theory of Group Presentations, London Mathematical Society lecture note series 42, Cambridge University Press: 9, 1980, ISBN 978-0-521-23108-4, MR 0695161
Tolstykh, Vladimir, What does the automorphism group of a free abelian group A know about A?, Blass, Andreas; Zhang, Yi (编), Logic and its Applications, Contemporary Mathematics 380, Providence, Rhode Island: American Mathematical Society: 283–296, 2005, MR 2167584, S2CID 18107280, arXiv:math/0701752, doi:10.1090/conm/380/07117
Ayoub, Raymond G.; Ayoub, Christine, On the group ring of a finite abelian group, Bulletin of the Australian Mathematical Society, 1969, 1 (2): 245–261, MR 0252526, doi:10.1017/S0004972700041496
Tolstykh, Vladimir, What does the automorphism group of a free abelian group A know about A?, Blass, Andreas; Zhang, Yi (编), Logic and its Applications, Contemporary Mathematics 380, Providence, Rhode Island: American Mathematical Society: 283–296, 2005, MR 2167584, S2CID 18107280, arXiv:math/0701752, doi:10.1090/conm/380/07117
Corner, A. L. S., Groups of units of orders in Q-algebras, Models, modules and abelian groups, Walter de Gruyter, Berlin: 9–61, 2008, MR 2513226, doi:10.1515/9783110203035.9. See in particular the proof of Lemma H.4, p. 36, which uses this fact.
Corner, A. L. S., Groups of units of orders in Q-algebras, Models, modules and abelian groups, Walter de Gruyter, Berlin: 9–61, 2008, MR 2513226, doi:10.1515/9783110203035.9. See in particular the proof of Lemma H.4, p. 36, which uses this fact.
Blass, Andreas, Injectivity, projectivity, and the axiom of choice, Transactions of the American Mathematical Society, 1979, 255: 31–59, JSTOR 1998165, MR 0542870, doi:10.1090/S0002-9947-1979-0542870-6. For the connection to free objects, see Corollary 1.2. Example 7.1 provides a model of set theory without choice, and a non-free projective abelian group in this model that is a subgroup of a free abelian group , where is a set of atoms and is a finite integer. Blass writes that this model makes the use of choice essential in proving that every projective group is free; by the same reasoning it also shows that choice is essential in proving that subgroups of free groups are free.
Tolstykh, Vladimir, What does the automorphism group of a free abelian group A know about A?, Blass, Andreas; Zhang, Yi (编), Logic and its Applications, Contemporary Mathematics 380, Providence, Rhode Island: American Mathematical Society: 283–296, 2005, MR 2167584, S2CID 18107280, arXiv:math/0701752, doi:10.1090/conm/380/07117
Ayoub, Raymond G.; Ayoub, Christine, On the group ring of a finite abelian group, Bulletin of the Australian Mathematical Society, 1969, 1 (2): 245–261, MR 0252526, doi:10.1017/S0004972700041496
Blass, Andreas, Injectivity, projectivity, and the axiom of choice, Transactions of the American Mathematical Society, 1979, 255: 31–59, JSTOR 1998165, MR 0542870, doi:10.1090/S0002-9947-1979-0542870-6. For the connection to free objects, see Corollary 1.2. Example 7.1 provides a model of set theory without choice, and a non-free projective abelian group in this model that is a subgroup of a free abelian group , where is a set of atoms and is a finite integer. Blass writes that this model makes the use of choice essential in proving that every projective group is free; by the same reasoning it also shows that choice is essential in proving that subgroups of free groups are free.
Tolstykh, Vladimir, What does the automorphism group of a free abelian group A know about A?, Blass, Andreas; Zhang, Yi (编), Logic and its Applications, Contemporary Mathematics 380, Providence, Rhode Island: American Mathematical Society: 283–296, 2005, MR 2167584, S2CID 18107280, arXiv:math/0701752, doi:10.1090/conm/380/07117