स्फुरतिस्फुलत्योर्निर्निविभ्यः

Operational rule vidhi

ष् is optionally the substitute of the स् of स्फुर् and स्फुल् after the उपसर्ग (Prepositions) निस् , नि and वि.

Word by word (padaccheda)

Target स्फुरति- स्फुलत्योः ṣaṣṭhī · dvivacanaLeft context निस्- नि- विभ्यः pañcamī · bahuvacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.

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

Full reading

स्फुरति-स्फुलत्योः निस्-नि-विभ्यः

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 स्फुर् and स्फुल् after the उपसर्ग (Prepositions) निस् , नि and वि।
Thus निष्ष्फुरति or निस्स्फुरति, निस्फुरति or निष्फुरति, विस्फुरति, विष्फुरति ॥ स्फुलति, निष्ष्फुलति, निस्स्फुलति, निस्फुलति, निष्फुलति, विस्फुलति, विष्फुलति ॥

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