Can Principal Ideals in Infinite Monoids Have Non-Empty Intersections?

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In summary, we discussed the possibility of proving that the intersection of two principal ideals xK and x'K is always equal to zero, given that x and x' are both elements of an infinite monoid A and not in a submonoid K. We also considered the additional hypothesis of requiring A to be a group or a specific property involving convolution operations. However, a counter-example was found using the free monoid on a singleton and it was suggested to focus on finding a property that satisfies the desired condition. One potential example was proposed involving the convolution operation in a monoid of functions.
  • #1
mnb96
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Hello,
I have an infinite monoid [itex]A[/itex] and a submonoid [itex]K[/itex].
let's assume I pick up an element [itex]x\in A-K[/itex],
now I consider the principal ideal of [itex]K[/itex] generated by [itex]x[/itex], that is [itex]xK=\{xk|k\in K\}[/itex].
The question is:
if I consider another element [itex]x'[/itex] such that [itex]x'\in A-K[/itex] and [itex]x'\notin xK[/itex], is it possible to prove that [itex]xK\cap x'K=0[/itex] ?

If that statement is not generally true, is there an additional hypothesis that I could make to force [itex]xK\cap x'K=0[/itex] hold?PS: I clicked too early and now I cannot change the title into something better.
 
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  • #2


I must admit that I have never heard of ideals in monoid theory, but just accepting your definition of xK I would say no.

Let [itex]A[/itex] be the free monoid on a singleton {y} so [itex]A=\{1,y,y^2,\ldots\}[/itex]. Let,
[tex]K = \{1,y^3,y^4,y^5,\ldots\}[/tex]
[tex]x = y[/itex]
[tex]x'=y^2[/itex]
It's trivial to verify [itex]x,x' \in A-K = \{y,y^2\}[/itex], [itex]x' \notin xK = \{y,y^4,y^5,\ldots\}[/itex] and:
[tex]xK \cap x'K = \{y^5,y^6,y^7,\ldots\}[/tex]

I don't immediately see an obvious property on A that would make it hold for arbitrary K except requiring A to be a group, or actually requiring exactly what you want.
 
  • #3


You are right. You easily found a counter-example.
I will now focus my interest in finding a property that satisfies that.

I don't know if the following is a valid example, but it is an attempt.
I was thinking about the set [itex]A[/itex] of functions [itex]f(x)[/itex] (plus the delta-function) with the operation of convolution [itex]\ast[/itex].
[itex](A,\ast)[/itex] should now be a monoid, and [itex]K[/itex] can be, for example, the submonoid of the gaussian distributions [itex]g(x)[/itex].
At this point if we assume that [itex]fK \cap f'K \neq 0[/itex] it means that there exists some gaussians [itex]g,g'\in K[/itex] such that [itex]f\ast g = f' \ast g'[/itex].

I haven't proved it yet, but intuitively it sounds strange that one could pick up an [itex]f'\notin fK[/itex] and get something equal to [itex]f \ast g[/itex] by just convolving. But maybe I am wrong?
 

FAQ: Can Principal Ideals in Infinite Monoids Have Non-Empty Intersections?

What is a principal ideal?

A principal ideal is an ideal in a ring that is generated by a single element. This means that every element in the ideal can be expressed as a multiple of the generator.

How is a principal ideal different from a non-principal ideal?

A non-principal ideal is an ideal that cannot be generated by a single element. This means that there is no single element that can represent all the elements in the ideal.

What is the significance of considering principal ideals?

Studying principal ideals is important in understanding properties of rings and their elements. It allows us to simplify calculations and proofs, and can also provide insight into the structure of a ring.

How are principal ideals used in algebraic number theory?

In algebraic number theory, principal ideals are used to study the properties of algebraic number fields and their rings of integers. They are particularly useful in proving the fundamental theorem of algebraic number theory.

Can every ideal be expressed as a principal ideal?

No, not every ideal can be expressed as a principal ideal. However, in certain rings, such as principal ideal domains, every ideal can be generated by a single element. In other rings, there may be a combination of elements that can generate the ideal.

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