The Viṣṇu Gāyatrī mantra is recited at the end of the Narāyaṇa Sūktam, which is presented in the Mahānārāyaṇa Upaniṣat. Who is Viṣṇu? Who is Nārāyaṇa? What, if any, are the similarities between Puruṣa of the Puruṣa Sūktam & Nārāyaṇa of the Nārāyaṇa Sūktam?