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There're a lot of examples of code for checking if a number is prime. Why don't people use Fermat's little theorem, i.e. this simple formula

$\qquad a^{p-1} \equiv 1 \pmod p$,

to check if a number is prime?

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    $\begingroup$ They do. But there exist non prime $p$ which satisfy that equation for some $a$, which needs to be worked around. It's all explained in wikipedia. $\endgroup$ – Karolis Juodelė Jan 19 '14 at 14:35
  • $\begingroup$ @KarolisJuodelė I think you should make this an answer (or close-vote since we don't strive to copy Wikipedia). $\endgroup$ – Raphael Jan 19 '14 at 15:30
  • $\begingroup$ In fact there are certain composite numbers, the co-called Carmichael numbers, which satisfy $a^{c-1}\equiv 1\pmod c$ for every $a$. The smallest example is 561. $\endgroup$ – Mark Dominus Jan 19 '14 at 16:01
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Expanding on Karolis's answer, the probabilistic primality tests used in practices, the Miller-Rabin test and the Solovay-Strassen test, are both based on modifications of the test you suggest, also known as the Fermat primality test. (Same for the less common Baillie-PSW test.) The problem with the Fermat test is that it is fooled by some composites known as Carmichael numbers.

The Solovay-Strassen test replaces the test $a^{n-1} \equiv 1 \pmod{n}$ with the similar formula $a^{(n-1)/2} \equiv \left(\frac{a}{n}\right) \pmod{n}$. Here $\left(\frac{a}{n}\right)$ is a Jacobi symbol, which for prime $n$ and $a \nmid n$ is equal to $1$ if $a$ is a quadratic residue modulo $n$ (i.e. has a square root modulo $n$) and to $-1$ if $a$ is a quadratic non-residue modulo $n$. There is a GCD-like algorithm for computing the Jacobi symbol, which makes the test feasible. It can be shown that if $n$ is an odd composite, then at least half the $a$s fail this test.

The Miller-Rabin test relies on the (similar) property that $1$ has two square roots modulo a prime, but more modulo composites. The idea is to check that $a^{n-1} \equiv 1 \pmod{n}$ and $a^{(n-1)/2} \equiv \pm 1 \pmod{n}$. If $a^{(n-1)/2} \equiv 1 \pmod{n}$, then we check that $a^{(n-1)/4} \equiv \pm 1 \pmod{n}$. And so on. The test fails if for some $k$, $a^{(n-1)/2^k} \equiv 1 \pmod{n}$ but $a^{(n-1)/2^{k+1}} \not\equiv \pm 1 \pmod{n}$. It can be shown that if $n$ is an odd composite, then at least three quarters of the $a$s fail this test (and so less iterations are needed compared to the Solovay-Strassen test).

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