8.3.40
नमस्पुरसोर्गत्योः
Operational rule vidhi
The विसर्ग of गतिसंज्ञक नमस् and पुरस् words is converted to सकार when followed by कवर्ग letter or a पवर्ग letter.
Word by word (padaccheda)
Target नमस्- पुरसोः ṣaṣṭhī · dvivacanaTarget गत्योः ṣaṣṭhī · dvivacana
- Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
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Full reading
नमस्-
Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.
Commentary
For the विसर्ग of नमस् and पुरस् there is substituted स् before a hard guttural or a labial when these words are गति -- पुरोऽव्ययम् 1.4.67 and साक्षात्प्रभृतीनि च 1.4.74.
Thus नमस्कर्त्ता, नमस्कर्तुम्, नमस्कर्त्तव्यम्; पुरस्कर्त्ता पुरस्कर्तुम्, पुरस्कर्त्तव्यम् ॥
Why do we say "when they are Gati"? Observe पूःकरोति, पुरौ करोति, पुरः करोति ॥ Here पुरः is a noun, Accusative Plural of पूः ॥
नमस् is Gati by 1.4.74, and पुरस् is Gati by 1.4.67. The anuvritti of अपदादौ ceases.
Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.