The king *re:g^-s

1)Are the only "survivors" of PIE *re:g^-s  Latin re:x, Sanskrit ra:j-an, Celtic ri:x, and perhaps Thracian Rhe:sos ?

 

2) Why did this word disappear among the other PIE people?

 3) Is this reflect any idea about the pre-history of Celts, Italics and Indians?

4) Is there any equivalent in another IE languages, another word that was equivalent to *re:g^-s ? Gothic thiudans (*teuto-nos)?

 

 

Joao SL