Martin Davis, Hilary Putnam, Julia Robinson, “The decision problem for exponential diophantine equations”, Matematika, 8:5 (1964), 69–79; Ann. of Math., 74 (1961), 425–436
2.
Julia Robinson, “Existential definability in arithmetic”, Matematika, 8:5 (1964), 3–14; Trans. Amer. Math. Soc., 72 (1952), 437–449
