The Paṇinian form is indeed dveṣāvahai as attested in the Siddhāntakaumudī under luṅi ca (SK 2434, A 2.4.43). I do not recall any special rule for vi + dviṣ, so the Paṇinian form would be vidveṣāvahai, and the form vidviṣāvahai would be a chāndasa usage where the lack of guṇa can either be explained by extending one of the bahulam rules to cover this case or as an ātmanepada usage from the nāmadhātu vidviṣa formed from the word vidviṣa (=vi + dviṣ + ka by igupadhajñāprīkiraḥ kaḥ, A 3.1.135) meaning enemy by sarvaprātipatikebhya ācāre kvibvā vaktavyaḥ (V 3.1.11).

MacDonnel/Whitney may have missed this form.

On 15 August 2016 at 03:17, Harry Spier <hspier.muktabodha@gmail.com> wrote:

Dear list members,

Apologies if this is an elementary question.

In the Shanti mantra taken from the Taittiriya Upanishad

saha nāvavatu saha nau bhunaktu saha vīryaṁ karavāvahai tejasvi nāvadhītam astu mā vidviṣāvahai
Why is it vidviṣāvahai and not vidveṣāvahai similar to the form dveṣāvahai ?
I couldn't see it listed as an irregular form in MacDonnels grammar for students or his Vedic grammar for students or Whitney.

Thanks,
Harry Spier

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