जायाया निङ्

Operational rule vidhi

For the final of जाया is substituted निङ् in a बहुव्रीहि समास .

Word by word (padaccheda)

Target जायायाः ṣaṣṭhī · ekavacanaSubstitute / term निङ् prathamā · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

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Full reading

जायायाः निङ्

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

As a rewrite rule (sketch from case endings)

जायायाः → निङ्

Replace jāyāyāḥ with niṅ. Inherited words (anuvṛtti) may add further conditions.

Commentary

For the final of जाया is substituted निङ् in a बहुव्रीहि समास ।
Thus युवतिर्जाया यस्य = युवजानिः (जाया + निङ् = जाय् + निङ् = जा + नि, the य् being elided by 6.1.66. So also वृद्धजानिः ॥ There is punvad-bhava also.

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