I should recall the notion of maximal subring of a commutative unitary ring $R$.



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> **Def:** A commutative ring $S$ is called a maximal subring of $R$ if $S \subset R$ and if $T \subset R$ constitute a commutative ring with the restricted addition and multiplication of $R$ and also $S\subsetneq T$ then we could deduce that $T=R$.
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I am interested in studing this notion in **Rings of continuous functions**.


We could easily deduce that for $x \neq y \in X$ The set of the form $$M\_{x,y}=\Big(f\in C(X): f(x)=f(y) \Big)$$ forms a maximal subring of $C(X)$


From the above summary and notations I could pose my Questions.



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> **Question1:** Is there a maximal subring in $C(X)$ other than all $M\_{x,y}$'s?
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> **Question2:** Is $X$ compact if all maximal subrings of $C(X)$ is of the form $M\_{x,y}$?
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PS:I suppose that all subrings of a commutative ring $R$ contains the unitary element of $R$.



functional subrings
Hi,
Yes, C(X) has very different maximal subrings. First note that the real line, say R, has uncountable many maximal subrings, see for example A. Azarang and O.A.S. Karamzadeh works about the existence of maximal subrings in Fields and Commutative Rings, Hence for any fixed maximal ideal M\_x of C(X), C(X) contains many maximal subrings different from the above. Also note that for any free maximal ideal M, C(X) contains a maximal subring which contains R.


