How to prove that (10^(2/3)-1)/(sqrt (-3)) is an algebraic integer

ormaybesaladqh

ormaybesaladqh

Answered question

2022-10-23

How to prove that 10 2 3 1 3 is an algebraic integer

Answer & Explanation

blackcat1314xb

blackcat1314xb

Beginner2022-10-24Added 19 answers

If we start with 3 x + 1 = 10 2 / 3 , we get
( 3 x + 1 ) 3 = 100
hence
100 = 3 3 x 3 9 x 2 + 3 3 x + 1.
Therefore,
3 3 x 3 + 9 x 2 3 3 x + 99 = 0.
Dividing through by 3 3 we obtain
x 3 + 3 3 x 2 x + 33 3 = 0
and rationalizing we get
x 3 + 3 3 3 x 2 x + 33 3 3 = 0
or
x 3 3 x 2 x 11 3 = 0.
This is a monic polynomial with coefficients in Z [ 3 ]; hence x is integral over Z [ 3 ], which in turn is integral over Z , so x is integral over Z , as desired.
( ( x 3 x ) 3 ( x 2 + 11 ) ) ( ( x 3 x ) + 3 ( x 2 + 11 ) ) = ( x 3 x ) 2 + 3 ( x 2 + 11 ) 2 = x 6 2 x 4 + x 2 + 3 x 4 + 66 x 2 + 363 = x 6 + x 4 + 67 x 2 + 363 ,
and we are done.

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