# Why is 1 not considered a prime number?

## Main Question or Discussion Point

Why is 1 not considered a prime number? It meets the requirement of being only divisible by itself and 1.

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AKG
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Because that's not the definition of a prime number. The definition of a prime number is:

A prime number (or prime integer, often simply called a "prime" for short) is a positive integer p > 1 that has no positive integer divisors other than 1 and p itself. (Source)

1 certainly does not meet that criteria since 1 is not greater than 1.

honestrosewater
Gold Member
Couldn't you also say n is prime iff n has exactly two factors? I like that better.

I was just going to ask about the reasons for excluding 1. Does it have something to do with coprimes?

no its just if 1 was a prime number then every other number would be a composite and prime which would defeat the purpose of calling it a prime list.
a prime is suppose to be unique in factorization 1*n...and even though 1 fits the purpose it would destroy the thought and terminology ...but yes you can think of 1 as being prime...its the fundamental number.

matt grime
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That 1 is not prime is purely a convention, and a modern one at that. it makes more sense for it not to be a prime. it isn't a composite either, it is a unit.

this kind of question, to my mind, fits in with the ones i get asked a lot like: but why do groups satisfy those 4 axioms. it's almost as if people believe that the axioms we choose are somehow god given, carved in some stone and we must make sense of these mysterious rules that came from nowhere when in fact they are man made.

MathematicalPhysicist
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matt grime said:
this kind of question, to my mind, fits in with the ones i get asked a lot like: but why do groups satisfy those 4 axioms. it's almost as if people believe that the axioms we choose are somehow god given, carved in some stone and we must make sense of these mysterious rules that came from nowhere when in fact they are man made.
i dont see any problem with god given axioms espcecially when "god" itself is man made definition.

matt grime
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some axioms are more believable than others (see the axiom of choice for instance for one that isn't) but there no absolute truths. we study, say, groups, not becuase someone one day from absolutely nowehre said ooh, these four axioms i wonder... but because the study of certain objects over time were unified as it was observed that they had common properties.

Icebreaker
1 being prime would invalidate the fundamental theorem of arithmetic.

matt grime
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invalidate? not the word i'd've chosen but then that may just be me being picky. the statement of the theorem is dependent upon us accepitng the definitions properly.

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shmoe
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neurocomp2003 said:
no its just if 1 was a prime number then every other number would be a composite and prime which would defeat the purpose of calling it a prime list.
This isn't true. Remove the "p>1" clause from the definition in AKG's post and the only change is 1 is now considered a prime (assuming you aren't going to quibble over the "1 and p itself" bit). It has no effect on any other number being prime or not.

It's just a convention that we use this definition. It has no effect on the mathematics behind any theorems, only in how we state them i.e. the fundamental theorem of arithmetic would not suddenly be wrong just irritating to state. It's turned out to be convenient to seperate primes from the units, so we build the definitions to take this into account.

From the above posts, it would seem that Occam's razor would mandate as extraneous the separate condition "p>1" for primes. honestrosewater's definition
n is prime iff n has exactly two factors
includes this condition, thus effecting parsimony.

I think it's kinda stupid not to see 1 as a prime.

How long has 1 not been considered a prime?

arildno
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WeeDie said:
I think it's kinda stupid not to see 1 as a prime.
Why?
Daminc said:
How long has 1 not been considered a prime?
From the beginning of the 19th century, I would believe, since that was approximately the time when mathematicians realized the need to take more care in what definitions they chose to use.

I see, it's such a long time ago that I learnt about primes that I couldn't remember if 1's were included then. Thanks for the clarification.

arildo: because the characteristic that makes primes interesting is the fact that they are only devisable by themself and 1. The number 1 serves this condition so I see no need to exclude 1 from the definition of primes. In fact, one might get off on a bad start if one were to exclude 1 and graph primes, in order to find connections between primes and other number series.

arildno
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No, the characterestic of a prime is that the whole number is greater than 1 and is divisible only with itself and 1. What you may find "interesting" is of little importance.

matt grime
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WeeDie said:
arildo: because the characteristic that makes primes interesting is the fact that they are only devisable by themself and 1. The number 1 serves this condition so I see no need to exclude 1 from the definition of primes. In fact, one might get off on a bad start if one were to exclude 1 and graph primes, in order to find connections between primes and other number series.
that isn't the proper definition of prime, it is your vwersin of the definition. and in any case it is better stated as "has exactrly two positive factors" since this precludes 1 (and even characterizes primes in the integers as well as the naturals).

arildno
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matt grime said:
that isn't the proper definition of prime, it is your vwersin of the definition. and in any case it is better stated as "has exactrly two positive factors" since this precludes 1 (and even characterizes primes in the integers as well as the naturals).
I assume that was directed at me.
Thanks for the correction.

HallsofIvy
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WeeDie said:
I think it's kinda stupid not to see 1 as a prime.
Do you consider the Fundamental Theorem of Arithmetic:
"Every positive integer can be written as a product of powers of primes in exactly one way"
stupid?

Calling 1 a prime would make it untrue since then we could write 6= 1*2*3 or 6= 12*2*3 or...

EL
matt grime said:
it is better stated as "has exactrly two positive factors" since this precludes 1 (and even characterizes primes in the integers as well as the naturals).
How do you get that the primes must be naturals out of this definition?

To me it seems like e.g. -7 is a prime from this (factors -7,-1,1,7...exactly two positive factors...)

matt grime
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erm yes i did just implicitly state the -7 is a prime in Z, and that is perfectly correct and is what the real definition of prime in an arbitrary ring tells us about primes in Z.

EL
matt grime said:
erm yes i did just implicitly state the -7 is a prime in Z, and that is perfectly correct and is what the real definition of prime in an arbitrary ring tells us about primes in Z.
Ah, I misunderstood your sentence. Thought you ment that your definition restricted the primes to be naturals, but now I see...(I can blame my bad English...).
I should have known better than trying to point out a mistake by the Math Guru!

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A prime number is an integer that has exactly two distinct factors.

EL