स्वतवान् पायौ

Operational rule vidhi

In the context of संहिता, when the word स्वतवान् is followed by the word पायु, the नकार of स्वतवान् is optionally converted to रुँ.

Word by word (padaccheda)

Word स्वतवान् Right context पायौ saptamī · ekavacana

Word Case not marked in the source data.
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

स्वतवान् पायौ

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

Commentary

रु is substituted for the न् of स्वतवान् before पायु।
As स्वतवाँ पा॒युर॑ग्ने॒ (Rig. IV. 2.6). The word is स्वतवस्, the नुम् is added by 7.1.83. The word is derived from तु वृद्धौ with the affix असुन् (स्वन्तवो यस्यऽसौ स्वतवान्) ॥

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