ivaṅ and uvaṅ are substituted for the i and u of a reduplicate, before a non-homogeneous vowel.,
Thus इयेष, उवोष, इयर्त्ति, 7.4.77 but ईषतुः and ईषुः, ऊषतुः and ऊषुः before homogenous vowels and इयाय and उवाय before a non-vowel.,
अभ्यासस्य असवर्णे
Adhyāya 6 · Pāda 4 · Rule 78