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I think it's worth questioning whether Gyula Y. is sufficiently notable for a WP page; it's not yet as clear-cut as it is for his father Gyula O. H. Katona. So let me explain my reasoning for creating the page:
He has a reasonable number of reasonably-well-cited research publications. Perhaps not enough yet to pass WP:PROF but enough that one can call him a successful mathematician.
His three textbook publications are to my mind an unusually large number, enough (together with the research) that I think he does pass WP:PROF.
Being part of a successful parent-child mathematician team (as with, say, the Bernoullis, or the Königs, or the Kuperbergs) is I think inherently notable.
The main reason I added him: having two Hungarian combinatorialists named Gyula Katona is very confusing. Enough so that I got information from the two of them mixed up when trying to create the page that ended up describing his father. I think having separate WP pages on both of them can serve a useful encyclopedic purpose, in clarifying the situation and thereby helping others avoid the same confusion.