Pāṇini sūtras

The affix ḍu comes after the verb bhū when it is preceded by the upasargas vi, pra and sam provided that the word to be formed does not mean an appellative.

1. vi+pra+sambhyaḥवि+प्र+सम्भ्यः

   Part	Lexeme	Sandhi form	Meaning	Stem + termination
   vव्	viवि	viवि	viवि	vव् + plus iइ
   pप्	praप्र	praप्र	praप्र	prप्र् + plus aअ
   sस्	samसम्	sambhyaḥसम्भ्यः	samसम्	samसम् + plus bhyaḥभ्यः

2. ḍuडु

   Part	Lexeme	Sandhi form	Meaning	Stem + termination
   ḍड्	ḍuडु	ḍuडु	ḍuडु	ḍड् + plus uउ

3. a+sañjñāyāmअ+सञ्ज्ञायाम्

   Part	Lexeme	Sandhi form	Meaning	Stem + termination
   aअ	aअ	aअ	not	aअ
   sस्	saṃjñसंज्ञ्	sañjñāyāmसञ्ज्ञायाम्	name	saṃjñसंज्ञ् + plus āyआय्

Anuvṛtti

1. bhuvaḥभुवः 3·2·179

   Part	Lexeme	Sandhi form	Meaning	Stem + termination
   bब्	bhūभू	bhuvaḥभुवः	bhūभू	bhभ् + plus uvaḥउवः