Does mean the image of g under i iterations of T? If so, are you sure it's not supposed to be ?
[Excerpt from Topics in Abstract Algebra Second Edition by I.N.Herstein pg 69]
This is what Herstein talks of verbatim:
Generally, if is a group, an automorphism of order of which is not an inner automorphism, pick a symbol and consider all elements , subject to if and only if , and . This way we obtain a larger group and group generated by cyclic group of order .
I have the following questions
1. What is the nature of the "a larger group " under discussion I mean the nature of the elements the operation.
2. As I understand this symbol that Herestein talks of abides to the binary operation of . Correct?
3. Is it correct to assume that where is the identity element.
4. I have difficulty imagining in but I guess I will come to that once I am clear on the behavior of as a group.
Thanks in advance.
~Kalyan.
Hi Tiny,
Yes stands for applying iteratively times on . I do agree that this notation is confusing. In fact if we take we end up with and on iteratively applying times on g we do get .
Here is a note I tried checking this section in Topics in Abstract Algebra by I.N.Herestein 3rd edition (edited by Barbara Cortzen and David J.Winter) and its omitted entirely. The preface does mention "certain typographical errors and notation inconsistencies in 2nd edition" probably it could be due to that. However the paragraph I have typed in is exactly same as it appears in the 2nd edition.
This is a description of one way to construct the semidirect product of G by the action of induced by the automorphism T.
A more concrete way to make this construction is to say that consists of pairs , with the group operation defined by (where the dotted plus means addition mod r).
In this construction, you can identify G with the (normal) subgroup . The connection with Herstein's construction is that corresponds to the element , where e is the identity element of G.
The purpose of this construction is that given the group G and the (outer) automorphism T, it embeds G in the larger group , in which the automorphism becomes inner (because , or in Herstein's notation ).
Hi Oplag,
Thanks for the reply it has at least opened a new dimension to think on. I am not able to grasp what you said completely as of now as I have a little understanding of direct products and semi direct product of a group. I will ping you again once I am clear on this.
Kalyan.