षट्कतिकतिपयचतुरां थुक्

Operational rule vidhi

When डट् follows, थुक् is the augment of the words 1. षट् 2. कति 3. कतिपय and 4. चतुर्.

Word by word (padaccheda)

Target - कति- कतिपय- चतुराम् ṣaṣṭhī · bahuvacanaSubstitute / term थुक् 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.

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

Full reading

-कति-कतिपय-चतुराम् थुक्

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)

- कति- कतिपय- चतुराम् → थुक्

Replace ṣaṭ-kati-katipaya-caturām with thuk. Inherited words (anuvṛtti) may add further conditions.

Commentary

When डट् follows, थुक् is the augment of the words 1. षट् 2. कति 3. कतिपय and 4. चतुर्।
The डट् is to be read into this sutra, and it should be changed into locative, as डटि 'when dat follows'. The word कतिपय is not a Numeral, it takes डट् by force of this sutra. Thus षण्णां पूरणो = षष्ठः 'sixth'. कतिथः 'the which in order?' कतिपयथः 'the one in order after several', चतुर्थः 'the fourth'.

Vart:- The affixes छ (ईय) and यत् come after चतुर् and there is elision of the first letter. As तुरीयः or तुर्यः ॥

In this and the two following sutras, the action of the ड् of डट् vanishes when an augment is added.

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