When is integral closure generated by one element

[coquelicot]

Hello,
This is not a homework problem, nor a textbook question. Please do not remove.
Is there a concrete example of the following setup :
R is an integrally closed domain,
a is an integral element over R,
S is the integral closure of R[a] in its fraction field,
S is not of the form R{[}b{]} for any element b in S.

For example, the integral closure of {\mathbb Z}(\sqrt 5) is the set of elements of the form (m + \sqrt 5 n)/2, where m^2 - 5n^2 is a multiple of 4. So, S is generated by \sqrt 5/2 over \mathbb Z: This does not fulfill the desired conditions.

 

[mathwonk]

i don't understand. in your example apparently R is not integrally closed. in fact under your conditions, that R is integrally closed and a is integral over R, this implies that R = R[a], doesn't it?

Oh sorry, you meant integral over it but in some finite field extension of its fraction field. so your R was Z and your a was sqrt(5). got it. and maybe you meant Z[sqrt(5)] instead of Z(sqrt(5))?

 

[coquelicot]

Yes, I meant {\mathbb Z}{[}\sqrt 5{]}.
In my example, R = {\mathbb Z},\ a = \sqrt 5 and S is the integral closure of {\mathbb Z}[\sqrt 5] in {\mathbb Z}(\sqrt 5), which has the form given above.

 

[mathwonk]

the answer is there does exist such a counter example Z[a] to the integral closure equaling Z[ b]. all your examples are numbers fields, and all number fields fall into your examples. You are asking for the structure of the ring of integers, which is known to have a finite basis as an abelian group. You want that basis to be the powers, up to some finite power, of a single integral element.
It is well known since Richard Dedekind that not all rings of integers in number fields have so called "power bases". here is a reference: (or just search on "non monogenic fields".)

http://wstein.org/129-05/final_papers/Yan_Zhang.pdf

 

[coquelicot]

Very useful. I was completely unaware of this topic. Thank you so many mathwonk.