The affix ḍu comes after the verb bhū 'to be' when it is preceded by the upasarga vi , pra and sam provided that the word to be formed does not mean an appellative.,
As विभुः 'all pervading'; प्रभुः 'lord'; सम्भुः 'Creator'.
Why do we say 'when it is not a proper name'? Observe विभूः 'a person called Vibhu'.
Vart:- The words मितद्रु &c, also are formed by this affix. As मितद्रु 'the sea'; शम्भुः 'causing happiness'.
Why have we applied डु and not क्विप् to this root, as the forms would have been the same in either case? We have done so for two purposes: (1) To prevent the rules relating to dhatus from applying to this. (2) To prevent the insertion of तुक् augment. Thus the Ist. dual of मितद्रु is मितद्रू, Ist. pl. is मितद्रवः । Here there is no substitution of उकङ् which the rule 6.4.77 required, if द्रु had retained its dhatu characteristic; so also this rule of accent in 6.1.175 does not apply to Instrumental sing. मितद्र्वा &c, (2.) We have no त् which rule (VI. I. 71) required. मितं द्रवति = मितद्रुः ।,