आग्रहायण्यश्वत्थाट्ठक्

Operational rule vidhi

The words आग्रहायण and अश्वत्थ get the प्रत्यय ठक् in the meaning of 'सा अस्मिन् पौर्णमासी इति संज्ञायाम्'.

Word by word (padaccheda)

Left context आग्रहायणी- अश्वत्थात् pañcamī · ekavacanaSubstitute / term prathamā · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

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

Pink letters are it-markers (anubandhas), tags dropped by 1.3.9. Hover or tap one for the rule that marks it, or try the it-letter finder.

Full reading

आग्रहायणी-अश्वत्थात् ठक्

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

Commentary

The affix ठक् comes in the sense of Locative -7th case after the words आग्रहायणी and अश्वत्थ ending in the 1st - Nominative Case-affix in construction and being the name of a full-moon night.
The words सास्मिन्पौर्णमासीति of the last sutra govern this also. This debars अण् ॥ Thus आग्रहायणिको मासः 'the month called Agrahayanika' i.e. in which the moon is full in the asterism of Agrahayana. Similarly आश्वत्थिकः ॥

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