∂³Σx²

Δ³Σx² was inscripted 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 name "∂³∑x²" 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.