The augment num is not added to radh before an affix beginning with the augment iṭ , except in the liṭ (Perfect Tense).,
As रधिता, रधितुम्, रधितव्यम्, but रन्धनम्, रन्धकः before Anit affixes, and ररन्धिव, ररन्धिम in the Perfect. When नुम् is added रन्ध् becomes a root ending in a conjunct consonant, and therefore the लिट् affixes after it are not कित् (असंयोगात् लिट् कित् 1.2.5), and therefore the न् is not elided by 6.4.24. But when the affix क्वसु of the Perfect is added, we have रेधिवस् Nominative Singular रेधिवान् ॥ Here the affix being expressly taught with an indicatory क of the न् is elided. The reduplicate is elided, the अ changed into ए, then इट् is added, then नुम्, and then the नुम् is elided by the expressly taught कित् ॥
Why was not the sutra made as इटि लिटि रधेः when by so doing, the augment नुम् would have been added only in the Perfect when it had इट् and no where else? This form of sutra would have also meant that नुम् would be added in that Perfect which took इट् and in no other Perfect, while cases other than Perfect might take it. That being so, there would be no नुम् in ररन्ध, while रधिता would require नुम् ॥ See, however, the Mahabhashya for the contra.,