Karnatakabhashabhushana 3.19.2

3.19.2

saṅkhyāsarvanāmaguṇavacanaviṣayē vartamānasya dantasya śabdasya tr̥tīyādivibhaktigaṇē parē aṟādēśō bhavati.

When a case inflection group beginning with the third follows, the substitute aṟ occurs in place of the final ending in u belonging to numerals, pronouns, and qualitative nouns.