How do I show that f(x)=1+2x+⋯+(p−1)x^(p−2) is not reducible on QQ, where p is prime.

Russell Marsh

Russell Marsh

Answered question

2022-10-01

How do I show that f ( x ) = 1 + 2 x + + ( p 1 ) x p 2 is not reducible on Q , where p is prime.

Answer & Explanation

Collin Gilbert

Collin Gilbert

Beginner2022-10-02Added 11 answers

By Gauss's lemma, we only need to prove that f is not irreducible over Z . We have
g ( x ) := ( x 1 ) 2 f ( x ) = ( p 1 ) x p p x p 1 + 1.
Consider g ( x ) in the field Z / p Z , where g ( x ) = ( p 1 ) ( x 1 ) p . Therefore, we have f ( x ) = ( p 1 ) ( x 1 ) p 2 in Z / p Z . Hence, f ( x + 1 ) = ( p 1 ) x p 2 over F p [ x ]. The constant term of f ( x + 1 ) is ( p 1 ) C 2 p p C 2 p 1 , which is divisible by p but not by p 2 . Then by Einsenstein's criterion, f ( x + 1 ) is irreducible. As a result, f ( x ) is irreducible.

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