Proving Z=HK: Group Theory Problem Homework

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SUMMARY

The discussion centers on proving that Z=HK for the groups H=<5> and K=<7> in group theory. The solution demonstrates that every integer n in Z can be expressed as a linear combination of elements from H and K, confirming that Z=HK. However, the intersection of H and K is <35>, indicating that Z is not the internal direct product of H and K. The justification for these claims is validated through the equation 3(5) - 2(7) = 1, which establishes the necessary conditions for the proof.

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Homework Statement


In Z, let H=<5> and K=<7>. Prove that Z=HK. Is Z the internal direct product of <5> and <7>?

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The Attempt at a Solution


Since 3(5)-2(7)=1, every integer n element of Z can be written as 3n(5)+(-2n)(7), and n is an element of HK( where group operation is addition and powers written as multiples). Therefore Z=HK. Since the intersection of H and K =<35>, then Z is not equal to H direct product K.
I think this is right but I am not sure on some of the justification of my claims. Can anyone help me with the justification?
 
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What part are you finding a hard time justifying? You've shown Z=HK and 5^2*7^2=35*35 so it's not a direct product. What else do you need?
 

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