नञ्दुःसुभ्यो हलिसक्थ्योरन्यतरस्याम्

Operational rule vidhi

The affix अच् comes optionally after the words 1. हलि and 2. सक्थि final in a बहुव्रीहि समास when preceded by a negative particle or by the words 1. दुस् and 2. सु.

Word by word (padaccheda)

Left context नञ्दुःसुभ्यः pañcamī · bahuvacanaTarget हलिसक्थ्योः ṣaṣṭhī · dvivacanaRight context अन्यतरस्याम् saptamī · 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).
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").

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Full reading

नञ्दुःसुभ्यः हलिसक्थ्योः अन्यतरस्याम्

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

Commentary

The affix अच् comes optionally after the words 1. हलि and 2. सक्थि final in a बहुव्रीहि समास when preceded by a negative particle or by the words 1. दुस् and 2. सु।
Thus अविद्यमाना हलिरस्य = अहलः or अहलिः, दुर्हलः or दुर्हलिः, सुहलः or सुहलिः ॥ So also असक्थः, असक्थिः, दुःसक्थः or दुःसक्थिः, सुसक्थः or सुसक्थिः ॥

Some read the text as हलिशक्त्योः ॥ According to this reading we have अशक्तिः or अशक्तः, दुःशक्तिः or दुःशक्तः (विरूपा शक्तिरस्य), सुशक्तिः or सुशक्तः ॥

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