8.4.61
उदः स्थास्तम्भोः पूर्वस्य
Operational rule vidhi
The स्था and स्तम्भ् are converted to पूर्वसवर्ण when they follow the upasarga उत् .
Word by word (padaccheda)
Left context उदः pañcamī · ekavacanaTarget स्था- स्तम्भोः ṣaṣṭhī · dvivacanaTarget पूर्वस्य ṣaṣṭhī · ekavacana
- Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
- Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
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Full reading
उदः स्था-
Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.
Commentary
After उद्, in the room of the स of स्था or स्तम्भ, the substitute is a letter belonging to the class of the prior (i.e. a dental is substituted for this स).
As उद् + स्थाता = उद् + थ्थाता = उद् + थाता 8.4.65 = उत् + थाता 8.4.55 = उत्थाता, उत्थातुम्, उत्थातव्यम् ॥ So also with स्तम्भ्, as, उत्तम्भिता, उत्तम्भितुम् &c. (See 1.1.67 and 1.1.54.
Why do we say of स्था and स्तम्भ? Compare उत्स्नात ॥
Vart:- In the Vedas, the above substitution takes place in the case of स्कन्द preceded by उद् ॥ As, अग्रे दूरमुत्कन्दः ॥
Vart:- So also when it means a disease. As उत्कन्दको नाम रोगः ॥ Or this form may have been derived from the root कन्द, and not स्कन्द ॥
Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.