While this only affects 5 U. S. Presidents, there is a URL at
https://familypedia.wikia.org/wiki/Famous_Descendants_of_William_th...
that shows how they are direct descendants of William the Conqueror. Unfortunately, there are about 20 names per screen and about 84 screens of names on this URL. It takes quite some time but you can locate each one. The number to the left of each name is the generation that person belongs to. If you are a blood relation of any one of the hundreds of people listed, you are a cousin to some degree to everyone on this list. If the name is printed in black, what you see is all the data the author had on them. If it is in blue, it's a hyperlink to a profile where lots more data may await you. If you are a direct descendant to someone here, you are directly descended from William the Conqueror and his great, great, great, grandfather Rollo (Norman: Rou, Rollo(u)n; Old Norse: Hrólfr; French: Rollon; c. 860 – c. 930 AD) was a Viking who became the first ruler of Normandy, today a region in northern France. He emerged as the outstanding warrior among the Norsemen who had secured a permanent foothold on Frankish soil in the valley of the lower Seine. After the Siege of Chartres in 911, Charles the Simple, the king of West Francia, granted them lands between the mouth of the Seine and what is now Rouen in exchange for Rollo agreeing to end his brigandage, swearing allegiance to him, religious conversion, and a pledge to defend the Seine's estuary from Viking raiders.
Obama is one of these presidents and is listed twice because there are two ways he's related. I hope this helps people.