क्तक्तवतू निष्ठा

The affixes क्त and क्तवतु are called निष्ठा।
This defines the nishtha affixes, that is to say, the affixes of the past participle. The actual affixes are त, तवत्, the क and उ being indicatory. They being कित् are liable to all the rules applicable to कित् affixes, such as rule 1.1.5. Thus we have, कृतः, कृतवान्, भुक्तः, भुक्तवान्.

The force of the indicatory उ in ktavatu is, by 4.1.6, to indicate that in forming the feminine of nouns ending in an affix having an indicatory vowel of uk pratyahara (उ, ऋ, लृ), the affix ङीप् (ई) must be added. Thus कृतवान् (masc.), कृतवती (fem.). The word nishtha occurs in sutra 7.2.14.