Is the pitāpūtrasamāgama-sūtra, a Mahayana sutra or does it occur in the nikayas or somewhere else? What is the historically accepted date of this sutra? Does this sutra precede Nagarjuna or is it a post-Nagarjuna sutra?
The Pitāpūtrasamāgama-sūtra is most likely based on some earlier Abhidharmic writings from one or two centuries after the Buddha’s death. Multiple versions of this sutra have been recovered, perhaps the most famous one from the Ratnakuta collection. The dating of this collection is uncertain, but the collection is mentioned in the Nikāyasaṅgraha, a Theravadin text. It states the collection was compiled by the Caityaka, which had branched off the Mahāsāṃghika school by the 1st or 2nd century BCE.
Nagarjuna lived somewhere between 150-250 CE, so the text very likely predates him.