The augment num is added after the vowel of the root rabh 'to begin' before an affix beginning with a vowel but not before the vikaraṇa śap or the affixes of the liṭ (Perfect Tense).,
Thus आरम्भयति, आरम्भकः, साध्वारम्भी, आरम्भमारम्भम्, आरम्भो वर्तते ॥ But आरभते in शप्, and आरेभे in लिट्, and आरब्धा before an affix beginning with a consonant.,