Fundamental theorem of arithmetic

In <a href="/facts/Mathematics/pxTouaz4">mathematics</a>, the fundamental theorem of arithmetic, also called the unique factorization theorem and prime factorization theorem, states that every <a href="/facts/Integer/PUwwrawx">integer</a> greater than 1 is prime or can be represented uniquely as a product of <a href="/facts/Prime_number/L20FLkgn">prime numbers</a>, <a href="/facts/Up_to/ydz4ztzX">up to</a> the order of the factors. For example,

1200
        =
        
          2
          
            4
          
        
        ⋅
        
          3
          
            1
          
        
        ⋅
        
          5
          
            2
          
        
        =
        (
        2
        ⋅
        2
        ⋅
        2
        ⋅
        2
        )
        ⋅
        3
        ⋅
        (
        5
        ⋅
        5
        )
        =
        5
        ⋅
        2
        ⋅
        5
        ⋅
        2
        ⋅
        3
        ⋅
        2
        ⋅
        2
        =
        …
      
    
    {\displaystyle 1200=2^{4}\cdot 3^{1}\cdot 5^{2}=(2\cdot 2\cdot 2\cdot 2)\cdot 3\cdot (5\cdot 5)=5\cdot 2\cdot 5\cdot 2\cdot 3\cdot 2\cdot 2=\ldots }

The theorem says two things about this example: first, that 1200 can be represented as a product of primes, and second, that no matter how this is done, there will always be exactly four 2s, one 3, two 5s, and no other primes in the product.
The requirement that the factors be prime is necessary: factorizations containing <a href="/facts/Composite_number/SPtW9ftC">composite numbers</a> may not be unique 
(for example, 
 
 
 
 12
 =
 2
 ⋅
 6
 =
 3
 ⋅
 4
 
 
 {\displaystyle 12=2\cdot 6=3\cdot 4}
 
).
This theorem is one of the main <a href="/facts/Prime_number/L20FLkgn">reasons why 1 is not considered a prime number</a>: if 1 were prime, then factorization into primes would not be unique; for example, 
 
 
 
 2
 =
 2
 ⋅
 1
 =
 2
 ⋅
 1
 ⋅
 1
 =
 …
 
 
 {\displaystyle 2=2\cdot 1=2\cdot 1\cdot 1=\ldots }

The theorem generalizes to other <a href="/facts/Algebraic_structure/TkuXca24">algebraic structures</a> that are called <a href="/facts/Unique_factorization_domain/aYsX2swC">unique factorization domains</a> and include <a href="/facts/Principal_ideal_domain/JCwAUnyW">principal ideal domains</a>, <a href="/facts/Euclidean_domain/ydwCD0fz">Euclidean domains</a>, and <a href="/facts/Polynomial_ring/ua3vk8Ih">polynomial rings</a> over a <a href="/facts/Field_(mathematics)/xAjAS4ko">field</a>. However, the theorem does not hold for <a href="/facts/Algebraic_integer/Qb0fD3R3">algebraic integers</a>. This failure of unique factorization is one of the reasons for the difficulty of the proof of <a href="/facts/Fermat%2527s_Last_Theorem/fHRc2t8z">Fermat's Last Theorem</a>. The implicit use of unique factorization in rings of algebraic integers is behind the error of many of the numerous false proofs that have been written during the 358 years between <a href="/facts/Pierre_de_Fermat/FLbUh75u">Fermat's</a> statement and <a href="/facts/Wiles%2527s_proof_of_Fermat%2527s_Last_Theorem/Xgr6wAf2">Wiles's proof</a>.

Fundamental theorem of arithmetic open-in-new

Fundamental theorem of arithmetic