au is substituted for the final of div before the affix su (Nominative 1st-Case and Vocative Case singular).,
As द्यौः ॥ There is a nominal-stem दिव् which is taken here. It has no indicatory letters annexed to it. The root दिव् is not to be taken here, as it has the indicatory letter उ and is exhibited in the Dhatupatha as दिवु ॥ The nominal-stem derived from दिवु, does not take औ, but ऊ, the Nominative Singular of which is द्युः as अक्षद्युः (See 6.4.19, and 6.1.131.,