negative prime numbers?

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negative prime numbers?

by helen » Tue Jan 30, 2007 10:48 am
Hopefully this will be a quick one, but does anyone know if negative numbers can be prime?

I'm thinking yes, but can someone please confirm? Thanks!

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Re: negative prime numbers?

by beatthegmat » Tue Jan 30, 2007 6:29 pm
helen wrote:Hopefully this will be a quick one, but does anyone know if negative numbers can be prime?

I'm thinking yes, but can someone please confirm? Thanks!
Nope. Prime numbers are only positive.

https://primes.utm.edu/notes/faq/negative_primes.html
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by Stacey Koprince » Wed Jan 31, 2007 11:57 am
This can trip people up b/c the technical definition would seem to allow negative numbers but, as Eric says, primes are only positive. The reason: the concept of prime was developed before the concept of negative numbers was developed. So primes were "by definition" positive b/c negative numbers didn't "exist" yet.

The easiest way to remember the definition for prime: prime numbers have exactly two factors. That helps you keep straight the pos/neg distinction, as well as remember that 1 is not prime while 2 is. :)
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by Ian Stewart » Fri Aug 08, 2008 11:26 pm
Stacey Koprince wrote:This can trip people up b/c the technical definition would seem to allow negative numbers but, as Eric says, primes are only positive. The reason: the concept of prime was developed before the concept of negative numbers was developed. So primes were "by definition" positive b/c negative numbers didn't "exist" yet.
It is not because primes were defined before negatives that negatives are not considered prime. Mathematicians easily adapted other concepts (evens/odds, for example) to negative numbers. Negatives are not considered prime because if they were, the Fundamental Theorem of Arithmetic (unique factorization into primes) would no longer be true. If -2 were prime, for example, we could 'prime factorize' 4 in two different ways: as 2^2 and as (-2)^2. If only positive numbers are considered prime, then every positive integer larger than 1 has one and only one prime factorization.

It is because the Fundamental Theorem of Arithmetic is truly fundamental to all of Number Theory that only positive numbers are considered prime.
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Negative 1

by gmatutor » Thu Aug 28, 2008 6:15 am
Of course for the GMAT all primes are positive.

However, negative one satisfies the Fundamental Theorem and the most common definition of a prime.

Positive one is not considered prime because it is divisible be exactly one integer rather than two.

Negative one on the other hand is divisible by exactly two integers (+1 and -1). Thus negative one satisfies the definition of a prime.

If we accept negative one as a prime then negative 2 does not satisfy the definition of the prime. It is divisible by 2, -1, -2, and 1. Since all negative integers are divisible by -1, the example with negative 2 above would apply to all negative integers less than -1. Thus the only possible negative prime is negative one.

Since that is the case, the Fundamental Theorem is satisfied if we do not let the prime factor negative one have a power other than one. In other words we cannot factor 5 into 5 X -1^2.

Thus a negative number such as - 120 can be broken down into a unique set of prime factors: 5 X 3 X 2^3 X -1.

This definition adds to the usefulness of the fundamental theorem allowing it to describe not only positive but also negative integers.

So why aren't negative numbers prime? It is because primes belong to a set of integers called natural numbers. Natural numbers are counting numbers. The first natural number is 1. So the definition of a prime is that it must first be a positive integer and then must be divisible by two, and only two, positive integers.

As for historically not considering negatives to be prime, the definition of primes has changed over time. The number one was considered prime throughout most of history.

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Re: Negative 1

by Ian Stewart » Thu Aug 28, 2008 9:25 am
gmatutor wrote:Of course for the GMAT all primes are positive.

However, negative one satisfies the Fundamental Theorem and the most common definition of a prime.

Positive one is not considered prime because it is divisible be exactly one integer rather than two.

Negative one on the other hand is divisible by exactly two integers (+1 and -1). Thus negative one satisfies the definition of a prime.
The most common definition of a prime is the following: 'primes are positive integers which are divisible by precisely two distinct positive integers.' If you remove the word 'positive' from the definition, the only primes would be 1 and -1 (which both have two integer divisors, 1 and -1). 3 would not be prime- it is divisible by 1, -1, 3 and -3.
gmatutor wrote: Since that is the case, the Fundamental Theorem is satisfied if we do not let the prime factor negative one have a power other than one. In other words we cannot factor 5 into 5 X -1^2.
Yes, but the restriction you need to impose on the powers of -1 here are similar to the restrictions as you would need to impose on the power on 1 in a prime factorization if you permitted 1 to be prime. Mathematicians do not permit 1 to be prime for this very reason; if 1 is not prime, the Fundamental Theorem of Arithmetic can be stated without any restrictions on powers.
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