Isomorphic Quotient Groups: A Counterexample

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SUMMARY

The discussion centers on the existence of isomorphic normal subgroups H and K within a group G, where the quotient groups G/H and G/K are not isomorphic. A key insight is that if H and K are conjugate subgroups, then G/H is isomorphic to G/K. Therefore, to find a counterexample, one must select non-conjugate normal subgroups and ensure that the index [G:H] allows for multiple groups of that order. This approach leads to a valid example demonstrating the stated condition.

PREREQUISITES
  • Understanding of group theory concepts, specifically normal subgroups.
  • Familiarity with quotient groups and their properties.
  • Knowledge of group isomorphism and conjugacy relations.
  • Basic comprehension of group indices and their implications.
NEXT STEPS
  • Research the properties of normal subgroups in group theory.
  • Study examples of non-conjugate subgroups within specific groups.
  • Explore the concept of group automorphisms and their effects on subgroup isomorphisms.
  • Investigate the implications of group indices on the structure of quotient groups.
USEFUL FOR

Mathematics students, particularly those studying abstract algebra, group theorists, and educators looking for examples of non-isomorphic quotient groups despite isomorphic normal subgroups.

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Homework Statement


Let H and K be normal subgroups of a group G. Give an example showing that we may have H isomorphic to K while G/H is not isomorphic to G/K.


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The Attempt at a Solution


I don't want to look in the back of my book just yet. Can someone give me a hint?
 
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If G and K are conjugates, then its easy to show that G/H and G/K are isomorphic, so you need to look for non-conjugate subgroups (more generally, this is true if there is an automorphism of G taking H to K). You also obviously need to pick [G:H] so that there is actually more than one group of this order.
 

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