Tachtsis, Eleftherios (2019), "The Urysohn lemma is independent of ZF + countable choice", Proceedings of the American Mathematical Society, 147 (9): 4029–4038, doi:10.1090/proc/14590, MR3993794
Tachtsis, Eleftherios (2019), "The Urysohn lemma is independent of ZF + countable choice", Proceedings of the American Mathematical Society, 147 (9): 4029–4038, doi:10.1090/proc/14590, MR3993794