नित्यं छन्दसि

Operational rule vidhi

Short ऋ is invariably substituted in the छन्दस् (Vedas) for the penultimate ऋ or ॠ of a Causative अङ्ग (stem) in the reduplicated Aorist.

Word by word (padaccheda)

Substitute / term नित्यम् prathamā · ekavacanaRight context छन्दसि saptamī · ekavacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").

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Full reading

नित्यम् छन्दसि

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

Commentary

Short ऋ is invariably substituted in the छन्दस् (Vedas) for the penultimate ऋ or ॠ of a Causative अङ्ग (stem) in the reduplicated Aorist.
As अवीवृधत्, अवीवृधताम् and अवीवृधन् ॥

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