अत आदेः

Operational rule vidhi

A long vowel is substituted in the लिट् (Perfect Tense) for the initial अ of the reduplicate.

Word by word (padaccheda)

Target अतः ṣaṣṭhī · ekavacanaTarget आदेः ṣaṣṭhī · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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

Full reading

अतः आदेः

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

Commentary

A long vowel is substituted in the लिट् (Perfect Tense) for the initial अ of the reduplicate.
This debars the single substitute of the form of the subsequent ordained by 6.1.97. Thus आट, आटतुः, आटुः ॥ Why do we say 'the initial'? Observe पपाच and पपाठ ॥ Thus अट् + अतुः = अट् अट् + अतुः = अ अट् + अतुः = आ अट् ॥

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