The question Help on Surjection, Injection, and Bijection basically consists of four different question, all of which have been answered on this site in the past. So it should be either closed for the reason the it contains multiple questions or as a duplicate. (I have even asked in chat which of the two possibilities is better, but I got no reply.)
I am aware that duplicate is not ideal choice, but it seemed to be less harsh on a new user and in a comment I have added an explanation that one post should not contain several questions together with link to the relevant discussion on meta.
I voted to close a duplicate. I hoped that other users will notice alternative links in comments and perhaps choose some other post as a duplicate target. (So that, ideally, after closing process the duplicate banner would contain links to some or all of the subquestions.) This did not happen, but at least the links are available in the comments and in the sidebar.
The post already has been through review queue and it has at the moment 4 close votes (as a duplicate), so one last vote is missing.