# Difference between revisions of "There exist subgroups of arbitrarily large subnormal depth"

From Groupprops

(New page: ==Statement== Let <math>k</math> be a positive integer. Then, we can find a group <math>G</math> and a subgroup <math>H</math> such that <math>H</math> is a <math>k</math>-[[subnormal sub...) |
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with each <math>H_i</math> normal in <math>H_{i+1}</math>, and there exists ''no'' series of length <math>k-1</math> with the property. | with each <math>H_i</math> normal in <math>H_{i+1}</math>, and there exists ''no'' series of length <math>k-1</math> with the property. | ||

+ | ==Related facts== | ||

+ | |||

+ | * [[Normality is not transitive]] | ||

+ | * [[Descendant not implies subnormal]] | ||

+ | * [[Ascendant not implies subnormal]] | ||

==Proof== | ==Proof== | ||

## Revision as of 15:12, 6 October 2008

## Statement

Let be a positive integer. Then, we can find a group and a subgroup such that is a -subnormal subgroup of but is *not* a -subnormal subgroup of . In other words, the subnormal depth of in is precisely . Equivalently, there exists a series of subgroups:

with each normal in , and there exists *no* series of length with the property.

## Related facts

## Proof

### Example of the dihedral group

`Further information: dihedral group`

Let be the dihedral group of order . Specifically, we have:

.

Let be the two-element subgroup generated by :

.

- is -subnormal in . Consider the series:

.

Each subgroup has index two in its predecessor, and is thus normal. The series has length , so is -subnormal in .

- is not -subnormal in : To see this, note that the above subnormal series is a subnormal series of minimum length, because, starting from the right, each subgroup is the normal closure of in the group to its right.