Praudhamanorama p227

p227

[On] "diva aut" (7.1.84): [On the text] "of the nominal stem": [That is,] of an underived one, or of one derived by the sūtra supplied in the Nyāsa, 'diverḍiviḥ', but not of the verbal root "divu krīḍādau", by the metarule: "In the mention of an element without an indicatory letter, an element with an indicatory letter is not comprehended". Therefore, it does not apply in akṣadyūḥ, etc. The letter t of aut is for the sake of pronunciation.
[On] sudyubhyām: Since the vowel u is a samprasāraṇa, lengthening is applicable by "halaḥ" (6.4.2), but does not occur due to the purposefulness of the attachment of the letter t in 'ut'. As for what was said by the older grammarian—'div optionally has substitution of ā before am'—that is disregarded, because it is not stated by the triad of sages and because it is useless. The form dyām, however, should be understood as belonging only to the word dyo. ... Thus end [stems] ending in v ...