अर्तिपिपर्त्योश्च

Operational rule vidhi

इ is substituted for the vowel of the reduplicate of ऋ and पृ in the reduplicated Present form.

Word by word (padaccheda)

Target अर्तिपिपर्त्योः ṣaṣṭhī · dvivacanaIndeclinable च avyaya

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Indeclinable An avyaya such as च "and", वा "optionally", न "not".

Dotted words are technical terms. Hover or tap one to see its card.

अर्तिपिपर्त्योः च

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

इ is substituted for the vowel of the reduplicate of ऋ and पृ in the reduplicated Present form.
As इयर्त्ति 6.4.78 धूमम्, पिपर्त्ति सोमम् ॥

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.