# Maximal subideal of an ideal

For a commutative ring $R$ with unity, I am looking for an equivalent condition for an ideal $T$ to have the property that $T$ contains a unique maximal proper subideal, equivalently, the sum of proper subideals of $T$ is not equal to $T$.

• Feb 12, 2018 at 7:46
• Also related: math.stackexchange.com/questions/2644665/… . In particular, this answer implies that if R is a Noetherian local ring, an ideal has precisely one maximal subideal if and only if it is a nonzero principal ideal. Feb 12, 2018 at 16:56
• I don't know if this is useful to you but a module for which "the sum of two proper submodules is not equal to the whole module" is called a hollow module, which is basically the dual notion to a uniform module. So you might be talking about a hollow ideals. Feb 12, 2018 at 18:38

Claim: Let $T$ be an ideal in a commutative ring. If $T$ contains a unique maximal proper subideal, then $T$ is principal.

Proof: Indeed take any element outside the unique maximal proper subideal. The ideal generated by that cannot be a proper subideal and hence has to be $T$.

So, this suggests that this is a very special property. For instance, that maximal subideal would have to contain the square of any generator of $T$ and if those generators are not zero-divisors, then they are associate (i.e. they can only differ by a unit multiple). If any generator is a zero-divisor then every element in $T$ is a zero-divisor. I think you can continue this line of thought to derive more special properties when this holds.

There is also a partial converse:

Claim: Let $T$ be a non-zero principal ideal in a commutative ring. Then every proper subideal of $T$ is contained in an ideal that is maximal among proper subideals in $T$.

(By the usual proof of existence of maximal ideals)

Proof: Let $T\subseteq R$ be a principal ideal generated by $t\in T$, $T'\subsetneq T$ a proper subideal, and consider the set of ideals $I\subseteq T$ such that $T'\subseteq I$, $t\not\in I$ which is the same as the set of proper subideals of $T$ containing $T'$. If there is a chain of such ideals $\ldots\subseteq I_\lambda\subseteq \dots$ with $\lambda\in \Lambda$, then $t\not\in J:=\cup_{\lambda\in\Lambda}I_\lambda$ is an upper bound of the chain and hence the statement follows by Zorn's lemma.

However, of course, this does not mean that there is a unique one...

On the positive side, here is a situation where what you are hoping for holds:

Claim: If $R$ is a DVR, then the ideals in $R$ form a totally ordered set.

Proof: This follows essentially from the definition of a DVR.

So, I suppose one can conclude that this holds sometime, but for most ideals in most rings it does not.

• Well to be entirely accurate one has to exclude the zero ideal from this... Feb 12, 2018 at 9:45
• @PeterHeinig the second statement "Claim" of the OP "Let $T$ be a principal ideal... then $T$ contains [...] proper subideals" is false for $T=0$. To say that $\{0\}$ is maximal among proper subideals of $\{0\}$ is not vacuously true, it is false, because it says in particular that $\{0\}$ is a proper subset of $\{0\}$, which means the existence of an element in the empty set.
– YCor
Feb 12, 2018 at 11:15
• Actually the standard conclusion of Zorn's lemma gives a correct statement without excluding anything: any proper subideal (of a principal ideal) is contained in a maximal proper subideal Feb 12, 2018 at 13:45
• @მამუკაჯიბლაძე: How do you get the upper-bound without the exclusion? Feb 13, 2018 at 17:37
• @მამუკაჯიბლაძე: I don't understand your argument. Are you saying that any subideal of a principal ideal is also principal? How about $R=k[x,y,z]$ and $(xy,xz)\subseteq (x)$? Feb 13, 2018 at 19:54