8.3.74
परेश्च
Operational rule vidhi
ष् is optionally the substitute of the स् of स्कन्द् after the उपसर्ग (Preposition) परि everywhere.
Word by word (padaccheda)
Left context परेः pañcamī · ekavacanaIndeclinable च avyaya
- Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
- Indeclinable An avyaya such as च "and", वा "optionally", न "not".
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Full reading
परेः च
Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.
Commentary
ष् is optionally the substitute of the स् of स्कन्द् after the उपसर्ग (Preposition) परि everywhere.
Thus परिष्कन्ता or परिस्कन्ता, परिष्कन्तुम् or परिस्कन्तुम्, परिष्कन्तव्यम् or परिस्कन्तव्यम् ॥
The word परि could well have been included in the last sutra as विपरिभ्यां स्कन्देर् &c. The very fact that it has not been so included, indicates that the prohibition of अनिष्ठायाम् does not apply to it. Thus परिष्कण्णः or परिस्कन्नः ॥
Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.