Is the Set of Products of Two Ideals Always an Ideal?

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In summary, we have shown that the set of products of elements of I and J, two ideals of a ring R, need not be an ideal. This is demonstrated through a counterexample using the polynomial ring R[x,y] and the ideals I and J with specific properties. This counterexample proves that the set of products of elements of I and J is not closed under addition, and therefore is not an ideal.
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gordon53
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Let I,J be ideals of a ring R. Show that the set of products of elements of I,J need not be an ideal (by counterexample - I have been trying to use a polynomial ring).
 
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  • #2
And how has it gone? What examples have you tried, and where has it gone wrong?
 
  • #3
I think I may have solved it now, and would appreciate confirmation or corrections:

Let R[x,y] be the ring of polynomials with real coefficients. Let I be the ideal containing all elements of C with zero constant term. Let J be the ideal containing all elements of C with zero constant term and zero coefficients of x, y and xy.

Then I has multiples of x, y, xy, x^2, y^2, etc.
And J has multiples of x^2, y^2, x*y^2, y*x^2, etc.

Now IJ contains x*x^2 = x^3 and y*y^2 = y^3.

But x^3 + y^3 is factorised uniquely (since to irreducible factors) as (x + y)(x^2 - xy + y^2). Neither of these polynomials is in J, and therefore the sum is not an element of IJ. So IJ is not closed under addition.
 

FAQ: Is the Set of Products of Two Ideals Always an Ideal?

What does it mean for something to be "ideal" in the scientific context?

In the scientific context, something that is considered "ideal" is a theoretical concept that is perfectly suited for its intended purpose. It may not exist in reality, but it serves as a standard or benchmark for comparison.

Why is it important to show that something does not have to be ideal?

By showing that something does not have to be ideal, we acknowledge the limitations and imperfections of our scientific understanding. This allows for more realistic and practical applications, rather than striving for an unattainable perfection.

How do scientists determine if something is ideal or not?

Scientists use a combination of theory, experimentation, and observation to determine if something meets the ideal standard. This process involves rigorous testing and analysis to determine the suitability and functionality of a concept or theory.

Can something still be considered successful if it is not ideal?

Yes, something can still be considered successful even if it is not ideal. Success is often measured by how well a concept or theory performs in real-world applications, rather than how closely it aligns with the ideal standard.

What are some examples of scientific concepts that do not have to be ideal?

There are many examples of scientific concepts that do not have to be ideal, such as the laws of thermodynamics, the theory of evolution, and the concept of gravity. These concepts have been widely accepted and used in practical applications, despite not being perfect or ideal.

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