Which of these is a prime number?
Answer7
7 has no divisors other than 1 and itself.
Science · Facts & stats
Prime facts & stats
7 fact-checked facts about prime numbers and number theory, each with the reason behind it. 5 more are in today's round and join this page after it closes.
The basics
Is 1 a prime number?
AnswerNo
1 has only one divisor, so it is not prime.
Going deeper
Twin primes are pairs of primes that differ by what?
Answer2
Examples include 11 and 13, or 17 and 19.
Which ancient method finds primes by crossing out multiples?
AnswerSieve of Eratosthenes
The sieve eliminates multiples of each prime.
What is a Mersenne prime?
AnswerA prime of the form 2^n − 1
Examples are 3, 7, 31 and 127.
Expert level
The Fundamental Theorem of Arithmetic says every integer above 1 has what?
AnswerA unique prime factorization
Each integer factors into primes in exactly one way, up to order.
Which unsolved conjecture says every even number greater than 2 is the sum of two primes?
AnswerGoldbach's conjecture
Goldbach proposed it in a 1742 letter to Euler.
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