I have posted a question about "Define a countably infinite subset of $X$
" at https://math.stackexchange.com/questions/2907637/define-a-countably-infinite-subset-of-x for several days and have not received any answer. I have some questions that have not got any answer such as this one, but I'm particularly in need of an answer for this question. Please help me check it out! Thank you for any help!