Math Problem Statement

For matrices A and B, where AB = BA (i.e., A and B commute), prove or disprove the following statement: (AB)^{10} = A^{10}B^{10}.

Solution

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Question 10 (16 pts): For matrices AA and BB, where AB=BAAB = BA (i.e., AA and BB commute), prove or disprove the following statement:

(AB)10=A10B10.(AB)^{10} = A^{10}B^{10}.


Let's go through the proof or disproof of the given statement.

Proof/Disproof:

Given that AA and BB are matrices and AB=BAAB = BA, the goal is to determine whether (AB)10=A10B10(AB)^{10} = A^{10}B^{10}.

First, consider the general property for any two commuting matrices AA and BB: (AB)n=AnBnif AB=BA.(AB)^n = A^nB^n \quad \text{if } AB = BA.

We can prove this by induction on nn.

Base Case (n=1n = 1): (AB)1=AB=A1B1.(AB)^1 = AB = A^1B^1. The statement holds true for n=1n = 1.

Inductive Step:

Assume the statement is true for some n=kn = k, i.e., (AB)k=AkBk.(AB)^k = A^kB^k. We need to show it holds for n=k+1n = k + 1.

Consider: (AB)k+1=(AB)kAB.(AB)^{k+1} = (AB)^k \cdot AB. Using the inductive hypothesis: (AB)k+1=(AkBk)AB.(AB)^{k+1} = (A^kB^k) \cdot AB. Since AA and BB commute, we have: (AkBk)AB=Ak(BkA)B=Ak(ABk)B=AkABkB=Ak+1Bk+1.(A^kB^k) \cdot AB = A^k \cdot (B^k \cdot A) \cdot B = A^k \cdot (A \cdot B^k) \cdot B = A^k \cdot A \cdot B^k \cdot B = A^{k+1} \cdot B^{k+1}.

Thus: (AB)k+1=Ak+1Bk+1.(AB)^{k+1} = A^{k+1}B^{k+1}.

By induction, the statement (AB)n=AnBn(AB)^n = A^nB^n holds for all positive integers nn.

Conclusion:

Since the above holds true for any integer nn, specifically for n=10n = 10: (AB)10=A10B10.(AB)^{10} = A^{10}B^{10}. Thus, the statement (AB)10=A10B10(AB)^{10} = A^{10}B^{10} is proven to be true.


Would you like further details or have any specific questions?

Here are some related questions:

  1. What other properties of commuting matrices can be derived?
  2. How does this proof change if the matrices AA and BB do not commute?
  3. What are some examples of matrices where ABBAAB \neq BA?
  4. How would this proof look in the case of (AB)n=AnBn(AB)^n = A^nB^n for n=2n = 2?
  5. Can we generalize this result to more than two matrices?

Tip: When working with matrices, always check if they commute, as many matrix identities and simplifications depend on this property.

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Math Problem Analysis

Mathematical Concepts

Matrix Algebra
Commutative Property

Formulas

(AB)^n = A^nB^n (if AB = BA)

Theorems

Commutativity in Matrices

Suitable Grade Level

Undergraduate Level (University Mathematics)