विदेः शतुर्वसुः

वसु is substituted for the Present Participle affix शतृ after the root विद् 'to know'.
Thus विद्वस् (nominative singular विद्वान्), विद्वांसौ, विद्वांसः ॥ The words formed with affixes having an indicatory उ or ऋ (उगित्) add a म् in their strong cases 7.1.70, and form their feminine with unaccented ई ॥ शतृ is an affix having an indicatory ऋ, therefore its substitute would also be considered as having an indicatory ऋ, the उ of वसु therefore is not absolutely necessary for the purposes of उगित् operations. वसु is so written, in order that in the sutra वसो संप्रसारणं 6.4.131, both क्वसु and वसु should be included. Nor can we object that in the above mentioned sutra, वसु with one indicatory letter being taken, cannot include an affix like क्वसु having two indicatory letters, -- on the maxim एकानुबन्धकग्रहणे न द्व्यनुबन्धकस्य -- for if that were so, there was no necessity of उ in वसु ॥ Some read the anuvritti of the word optionally into this : and we have विदन्, विदन्तौ, विदन्तः formed with शतृ (see 5.4.38).