विभाषा स्वसृपत्योः

Operational rule vidhi

The Genitive 6th-Case-affix is optionally elided after a stem ending in ऋ when 1. स्वसृ and 2. पितृ follow.

Word by word (padaccheda)

Substitute / term prathamā · ekavacanaRight context स्वसृ- पत्योः saptamī · dvivacana

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

स्वसृ-पत्योः

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

Commentary

The Genitive 6th-Case-affix is optionally elided after a stem ending in ऋ when 1. स्वसृ and 2. पितृ follow.
Thus मातुःस्वसा or मातुःष्वसा or मातृष्वसा, पितुःस्वसा or पितुःष्वसा or पितृष्वसा ॥ When the affix is elided, then स is invariably changed to ष by 8.3.85; and when it is not elided, then the change is optional 8.3.85. So also दुहितुः पतिः or दुहितृपतिः, ननान्दुः पतिः or ननान्दृपतिः ॥

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