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Prime numbers

Testing for primality by trial division up to the square root.

A prime has no divisors except 1 and itself. To test n, try the primes up to √n only: if n had a factor larger than √n, it would also have one smaller. For enormous numbers this is too slow and probabilistic tests take over, but the idea is the same.

Àwọn ààtò àwọn ìṣàmúlò-ètò · is 97 prime

Number-Theory · Prime-Numbers

Is 97 prime

Àwọn ìṣàmúlò-ètò

\sqrt{97} \approx 9
Only primes up to √97 need testing: any factor above it pairs with one below.
97 \bmod 2 = 1
2 does not divide 97.
97 \bmod 3 = 1
3 does not divide 97.
97 \bmod 5 = 2
5 does not divide 97.
97 \bmod 7 = 6
7 does not divide 97.
\text{97 is prime}
No prime up to the square root divides it, so it has no factors other than 1 and itself.

Fi àwọn àgbèwọlé hàn

\text{97 is prime}

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Bii o ṣe le: Prime numbers

Only primes up to √97 need testing: any factor above it pairs with one below.
2 does not divide 97.
3 does not divide 97.
5 does not divide 97.
7 does not divide 97.
No prime up to the square root divides it, so it has no factors other than 1 and itself.

Àwọn Àtòjọ-ẹ̀yàn

Why are primes so important?

Every integer factors into primes in exactly one way, so primes are the atoms of multiplication. Cryptography relies on that factoring being easy to state and hard to do.

How do I tell whether a big number is prime?

Trial division up to the square root works for small numbers. For large ones, probabilistic tests (Miller-Rabin) give an answer that is wrong with negligible probability, and deterministic tests (AKS) exist but are slower.

Diẹ̀ nínú Number Theory

Prime factorisation GCD and LCM Modular arithmetic Divisors Sequences Number bases Diophantine equations Fermat's little theorem and Euler's theorem RSA: cryptography from number theory