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May 8th, 2017, 06:23 PM  #1 
Banned Camp Joined: Apr 2016 From: Australia Posts: 244 Thanks: 29 Math Focus: horses,cash me outside how bow dah, trash doves  An example of a non rational algebraic integer
Hi I was hoping someone could give me an example of a non rational algebraic integer please

May 8th, 2017, 07:49 PM  #2  
Senior Member Joined: Aug 2012 Posts: 2,311 Thanks: 706  Quote:
Also here is a nice explanation. They give the example that $4 + \sqrt{7}$ is an algebraic integer but $\frac{4 + \sqrt{7}}{2}$ isn't. I didn't work out the details but offhand it looks like the latter is a root of a polynomial like $2x^2$something which is against the rules for algebraic integers. The algebraic integers are the roots (in the complex numbers, which includes the reals) of all the monic polynomials with integer coefficients. Monic means the leading coefficient is $1$.  
May 9th, 2017, 11:29 PM  #3 
Banned Camp Joined: Apr 2016 From: Australia Posts: 244 Thanks: 29 Math Focus: horses,cash me outside how bow dah, trash doves 
Thanks, so Eisenstein integers https://en.wikipedia.org/wiki/Eisenstein_integer would be a subring of this ring if I'm correct, my basic goal for this exercise is to find the neatest clearcut means of defining the entire ring, which I'm guessing would just be a set declaring the necessity of all of its elements being roots of a monic poly with integer coefficients. Thanks anyway, it just case of brain freeze having to readapt my definition of an integer to include those on the complex plane! Last edited by skipjack; May 9th, 2017 at 11:58 PM. 

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algebraic, integer, rational 
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