Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Understanding Permutations

  1. Oct 16, 2003 #1
    Hello,

    I am having trouble understanding permutations. Here is the definition that I was given:

    Let A be a nonempty set, A permutation of A is any function [alpha]: A --> A such that [alpha] is both one-to-one and onto.

    Then the example:

    Let A = {1,2}. Then the permutations of A are

    1[alpha] = 1
    2[alpha] = 2
    1[beta] = 2
    2[beta] = 1

    Now I know this is basic, basic stuff. And many of you will say "What the heck?!!? Just think about it!" But I have thought about it. And for a long time. I just have trouble with some really elementry subjects sometimes. I was hoping if someone just rephrases it for me maybe I will understand it a bit better. I am mostly having trouble understanding the example. But perhaps I am not understanding the definition as well. For if I did I would understand the definition. [?]

    Here is what I know about one-to-one, onto functions.

    I know that for a function to be one-to-one, every x that is an element of the set A, there should be an x[alpha] which is also an element in A.

    And for a function to be onto, for every x[alpha] that is in A, there should be an x which is an element of A.

    (PLEASE correct me if my definitions are incorrect. OR elaborate on the definitions if you deem them insufficient.)


    Now on an intuitive level, I "THINK" I know what a permutation is - any possible combo of the elements in A. NOTE: I didn't say that my interpretation is correct.

    Now, for the first two parts of the example

    1[alpha]=1
    2[alpha]=2

    I interpret this to be some function such as sort of an identity function.

    With the second two parts of the example, I SEE what is going on, but I can't explain it. I see that put an input x into a function and get y, put y into the same function and get x .

    If anyone has the time or patience to SPOONFEED me on this definition I would appreciate it greatly.

    Note: I am also aware that given n elements of a set there are n! combinations of these elements.

    Sincerely,
     
    Last edited by a moderator: Oct 17, 2003
  2. jcsd
  3. Oct 17, 2003 #2
    Alright. Nevermind my garbage up there. I finally figured it out. It only took about five to six hours but I was able to get it.

    Thanks to all who took the time to browse my post.

    Cheers.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Understanding Permutations
  1. Permutation mapping (Replies: 4)

  2. Multiplying permutations (Replies: 10)

  3. Permutation Group (Replies: 29)

Loading...