Query regarding the academic journal submission Asking on behalf of my friend.
He had worked on some problems in the summer of 2018 when he was a student at some X university. After a year, he again started working on the same problem and will be going to publish it in some journal. But now he joined university Y.
He has this query: what author's address and mail he needs to mention on the paper? Or what else he needs to do?
PS: The funding for the summer internship was given by university X.
Thanks
You want to provide maximal useful information. That should include funding sources and a current address.
Your friend should provide the journal with all that information. They will decide how to use it. A paper I wrote years ago needed three affiliations. It started with

The footnote read

That was in the days before email. Now there would be a current email address.