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In the question Proof of no prime-representing polynomial in 2 variables, Alon Amit asks if Ribenboim's claim that a prime-representing polynomial (a Diophantine polynomial in which the positive values are precisely the primes) must have at least three variables has been proven. Alon suggested that perhaps the number was a typo, that all that is known is that (trivially) no univariate polynomial is prime-representing.

As of Jones 1982 [1, p. 550] the question of the existence of a universal Diophantine equation in two variables was open, so certainly it was not known that the number of variables for the special case of the primes was more than 2 at that time.

[1] James P. Jones, "Universal Diophantine equation", The Journal of Symbolic Logic 47:3 (1982), pp. 549-571.

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Davis [1] writes that the two-variable case of universal Diophantine equations is still open as of 2006. (Ribenboim's book was published in 1996.) So the question of a prime-representing polynomial in two variables was (and, presumably, is) still open.

[1] Martin Davis, [FOM] Decidability of Diophantine equations, post to the FOM mailing list, December 14 2006.

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