kat is substituted for ku in a tat-puruṣa samāsa when a word beginning with a vowel follows as the second member.,
As कदजः, कदश्वः, कदुष्ट्रः, कदन्नम् ॥ Why do we say in a Tatpurusha? Observe कूष्ट्रो राजा ॥ Why do we say \when the second member begins with a vowel\? Observe कुब्राह्मणः, कुपुरुषः ॥
Vart:- कत् is substituted before त्रय, as कत्रयः = कुत्सितास्त्रयः ॥,