The final vowel of pitṛṣvasṛ is elided when the affix ḍhak , in the sense of a descendant, is added.,
Thus पितृष्वस् + ढक् = पैतृष्वसेयः ॥ This sutra indicates by jnapaka that the word पितृष्वसृ takes the affix 'dhak' also.,
ठकि लोपः
Adhyāya 4 · Pāda 1 · Rule 133