The proof by Pythagoras (or more likely one of his students) about 500 BCE has had a profound effect on mathematics. It shows that the square root of 2 cannot be expressed as the ratio of two integers (counting numbers). The proof bifurcated "the numbers" into two non-overlapping collections—the rational numbers and the irrational numbers. This bifurcation was used by Cantor in his diagonal method, which in turn was used by Turing in his proof that the Entscheidungsproblem, the decision problem of Hilbert, is undecidable.
It is unknown when, or by whom, the "theorem of Pythagoras" was discovered. The discovery can hardly have been made by Pythagoras himself, but it was certainly made in his school. Pythagoras lived about 570–490 BCE. Democritus, born about 470 BCE, wrote on irrational lines and solids ...
— Heath,[citation needed]
Proofs followed for various square roots of the primes up to 17.
There is a famous passage in Plato's Theaetetus in which it is stated that Teodorus (Plato's teacher) proved the irrationality of
{\displaystyle {\sqrt {3}},{\sqrt {5}},...,}\sqrt{3}, \sqrt{5}, ...,
taking all the separate cases up to the root of 17 square feet ... .[7]
A more general proof now exists that:
The mth root of an integer N is irrational, unless N is the mth power of an integer n".[8]
That is, it is impossible to express the mth root of an integer N as the ratio a⁄b of two integers a and b, that share no common prime factor except in cases in which b = 1.