Udāvasu (Udavasu) was the son of Rājarṣi (Rajarshi) Janaka, in the lineage of Nimi, son of Ikṣvāku (Ikshvaku). Udāvasu had a son named Nandivardhana.

Viṣṇupurāṇa(4.5.12)
Vāyupurāṇa(89.6)
Brahmāṇḍapurāṇa (Maharṣi)(2.64.6)
Bhāgavatpurāṇa Vols. 1-12(9.13.14)
Rāmāyaṇa (Tarkaratna-Beng.)(1.71.4-5)