रुग्रिकौ च लुकि

Operational rule vidhi

The augments come after the reduplicate of a root which has ऋ at the penultimate position, only when the यङ् of the Intensive is elided.

Word by word (padaccheda)

Substitute / term रुग्रिकौ prathamā · dvivacanaIndeclinable च avyayaRight context saptamī · ekavacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".
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

रुग्रिकौ च

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

Commentary

The augments come after the reduplicate of a root which has ऋ at the penultimate position, only when the यङ् of the Intensive is elided.
Thus नर्नत्ति, नरिनर्ति, and नरीनर्ति; वर्वर्ति, वरिवर्ति, वरीवर्ति ॥ The उ in रुक् is for the sake of pronunciation, the augment is र् ॥

Vart:- मर्मृज्यते and मर्मृज्यमानासः should also be enumerated. These have taken रुक् augment, though the यङ् is not elided here.

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