There is an interesting web site called Mathematical Genealogy http://genealogy.math.ndsu.nodak.edu/ which lists about 140000 mathematicians with their student-advisor relations. I think that this site can be useful for historian of sciences in many respects. Of course, extension of the modern student-advisor relation to the past is anachronistic, and many people listed there can be hardly called mathematicians, but nevertheless the site contains a lot of interesting information.
According to it, the genealogy of most modern mathematicians can be traced back to 14-th century, to several people who apparently moved to Western Europe from Constantinopole when it was conquered by the Turks. Their names are Manuel Bryennios, Demetrios Kydones, Elissaeus Judaeus and few others. 119K (of the total 142K) of modern mathematicians have this Elissaeus as their ancestor. I am trying to find out
What exactly is known about these people ?
And where the information comes from? Internet search did not give much, though they are frequently mentioned in mathematicians web sites with the reference on this Genealogy. But where the data of this Genealogy for 14-th century come from, from what sources?
EDIT. On the answer of Mauro ALLEGRANZA. I understand that the "sources" he cites is Wikipedia. Of course I looked into Wikipedia BEFORE asking this question:-) A "totally unknown" teacher of Gemistos I did not find in Wikipedia. So I wonder: where the information about him comes from? On what grounds was it included to Math Genealogy?
I also understand all that these people were not mathematicians but rather clerics or philosophers. The real question is: what is the source of the information in Math Genealogy, and how reliable this source is.
EDIT2. I check my "genealogy" periodically (once a year in the average) and it expands:-) Now I see some ancestors which lived at the same time but look independent (according to the Math Genealogy) from those who came from Constantinopole. For example, Johannes Stoffler, who was born a year before the fall of Constantinopole, and has no known "PhD adviser".