When Does (ab)^n Equal (a^n)(b^n) in Ring Theory?

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SUMMARY

In ring theory, the equation (ab)^n = (a^n)(b^n) holds true under specific conditions, primarily when the elements a and b commute, i.e., ab = ba. The discussion highlights that for n = 2, the equality can be expressed as abab = aabb, leading to the conclusion that a(ba - ab)b = 0. This condition is not universally applicable, especially in non-commutative rings such as rings of matrices, where the equality may not hold.

PREREQUISITES
  • Understanding of ring theory concepts, particularly commutative and non-commutative rings.
  • Familiarity with algebraic structures and properties of multiplication in rings.
  • Knowledge of positive integers and their role in exponentiation within algebraic contexts.
  • Basic understanding of matrix algebra and its implications in ring theory.
NEXT STEPS
  • Research the properties of commutative rings and their implications on multiplication.
  • Study the implications of non-commutative rings, particularly in the context of matrix multiplication.
  • Explore the concept of zero divisors in rings and their role in equations like a(ba - ab)b = 0.
  • Learn about the generalization of the binomial theorem in non-commutative settings.
USEFUL FOR

This discussion is beneficial for students and researchers in abstract algebra, particularly those studying ring theory, as well as mathematicians exploring the properties of commutative and non-commutative structures.

gunnygunz
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Homework Statement


Let R be a ring and a,b be elements of R. Let m and n be positive integers. Under what conditions is it true that (ab)^n = (a^n)(b^n)?

Homework Equations


The Attempt at a Solution

We must show ab = ba.

Suppose n = 2.

Then (ab)^2 = (ab)(ab) = a(ba)b = a(ab)b = (aa)(bb) = (a^2)(b^2).

I am not sure where to go from here...
 
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First of all, you said "let m and n be integers," but m was never mentioned again. Is there a typo somewhere?

Second: You said "We must show ab = ba". But then you ASSUMED it was true in this step: a(ba)b = a(ab)b. This is not necessarily true unless the ring is commutative, in which case this problem becomes trivial.

I would start with the n = 2 case. What must be true in order for (ab)^2 to equal a^2 b^2? This is equivalent to writing

abab = aabb

or

a(ba - ab)b = 0

Clearly this is true if ab = ba, but is this necessary? What if this is a ring of matrices, for example?
 
Last edited:

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