Ircher:
Elements:
TemporalLich :
Takes 2 to lynch.
EBWOPIn post 28, Ircher wrote:Whoever proves the sqrt of two is irrational first to my satisfaction gains immunity from me.
Proof by unique factorization
An alternative proof uses the same approach with the fundamental theorem of arithmetic which says every integer greater than 1 has a unique factorization into powers of primes.
Assume that √2 is a rational number. Then there are integers a and b such that a is coprime to b and √2 = a/b. In other words, √2 can be written as an irreducible fraction.
The value of b cannot be 1 as there is no integer a the square of which is 2.
There must be a prime p which divides b and which does not divide a, otherwise the fraction would not be irreducible.
The square of a can be factored as the product of the primes into which a is factored but with each power doubled.
Therefore, by unique factorization the prime p which divides b, and also its square, cannot divide the square of a.
Therefore, the square of an irreducible fraction cannot be reduced to an integer.
Therefore, √2 cannot be a rational number.
This proof can be generalized to show that if an integer is not an exact kth power of another integer then its kth root is irrational. For a proof of the same result which does not rely on the fundamental theorem of arithmetic, see: quadratic irrational.