∂³Σx²

∂³Σx² was inscribed on K9's regeneration unit. (TV: Regeneration) Similar inscriptions were seen by the First, Second and Third Doctors in the tomb of Rassilon. (TV: The Five Doctors)

Behind the scenes
This is close to the Doctor's real name "∂³Σx²" as given according to the 1972 Doctor Who reference guide The Making of Doctor Who, which is, however, not a valid source on this wiki. It was also said to be his real name in an article in Marvel Premiere #57.