This question already has an answer here:
Consider p such that
PrimeQ[p] == True. How do I compute n such that
Prime[n] == p?
In other words, what is the inverse function of "Prime"?
As a concrete example, consider
p to be the first prime that factorizes rsa-768,
p = rsa768a = 33478071698956898786044169848212690817704794983713768568912431388982883793878002287614711652531743087737814467999489
PrimeQ[rsa768a] is True; I want to know its n.
Trying Jason's suggestion,
InverseFunction[Prime][rsa768a], does not return the expected result (for a small prime it does). Trying LLlAMnYP suggestion,
PrimePi::largp: Argument ... in PrimePi[...] is too large for this implementation. >>