Euler's totient theorem

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In number theory, Euler's totient theorem (also known as the Fermat–Euler theorem) states that if n and a are coprime, (meaning that the only number that divides n and a is 1), then the following equivalence relation holds:

where is Euler's totient function.

Euler's theorem is a more refined theorem of Fermat's little theorem, which Pierre de Fermat had published in 1640, a hundred years prior. Fermat's theorem remained unproven until the work of 18th-century Swiss mathematician Leonhard Euler.