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Thread: inverse function

  1. #1 inverse function 
    Forum Ph.D. Heinsbergrelatz's Avatar
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    how do you tell if a specific function has an inverse or not?


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  3. #2  
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    Er, by showing it's a bijection. But one needs to know what the function IS.

    Look, this comes up from time-to-time here, and (I believe) confusion is caused by some horrid, but conventional, notation. So let's step through it.

    First, without any rigour, define an injection by: for any there is at most one such that .

    Now, with the same lack of rigour define a surjection as: for any there is at least one such that .

    Roll these together, and you have the bijection: for any there is at least one and at most one such that .

    Now. The "horrid" notation I referred to above arises as follows: no function, on sets or otherwise, is allowed to have multiple images (these being the result of applying our function to its domain, here ). But, by our surjection above, the preimage MUST be a non-empty set, say .

    The preimage of our injection is a set also. So, by my above, for the injection , then either or . This called a "singleton set"

    For the bijection we will therefore have that .

    In this particular circumstance is customary to ever so slightly abuse the notation, and to equate and by a further slight abuse to call the inverse of


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  4. #3 Re: inverse function 
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    Quote Originally Posted by Heinsbergrelatz
    how do you tell if a specific function has an inverse or not?
    What Guitarist said is correct. But perhaps a bit difficult to follow if you have not seen this before.

    In simpler language, f is invertible if given f(x) you can figure out what x is.

    Now, strictly speaking this only really means that f is injective or one-to-one, but any injective function is invertible if you restrict the range to the image thereby making the function surjective by fiat.
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  5. #4  
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    so basically e.g should equal
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  6. #5  
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    Well, you need to specify your function. So, for example, if , then yes, this true.

    But if, as is equally possible without this specification, that then it is false, since and also then the preimage .

    This last is a SET, as I tried to explain. DrRocket suggested you may not understand my post; I found it hard to believe at the time, but now I see he must be right.
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  7. #6  
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    the example i gave was where the function has an inverse. then more generally speaking;

    a function defined should equal to in order for it to have an inverse?

    This last is a SET, as I tried to explain. DrRocket suggested you may not understand my post; I found it hard to believe at the time, but now I see he must be right.
    i didnt really read your post, (no offense) because such terms mentioned by your post, i didnt quite yet encounter. therefore, instead i read Dr. Rockets explanation which was alittle more concise and easier to understand. but now you said that "quoted by me", i do understand what you are trying to explain.
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  8. #7  
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    Quote Originally Posted by Heinsbergrelatz
    i didnt really read your post, (no offense)
    Yes, GREAT offense, as it seems I wasted my precious time
    because such terms mentioned by your post, i didnt quite yet encounter.
    Then you only had to ask, and we could get a discussion going which may been of interest to ALL our readers.
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  9. #8  
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    Yes, GREAT offense, as it seems I wasted my precious time
    sorry for making you waste your time, but i did read it eventually, though not all terms i could understand. But usually i seldom read posts that i hardly understand, or not yet learned. just a habit i guess, but since it offended you , my apologies.

    Then you only had to ask, and we could get a discussion going which may been of interest to ALL our readers.
    yes, that would have been much better. too bad
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  10. #9 Re: inverse function 
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    Quote Originally Posted by Heinsbergrelatz
    how do you tell if a specific function has an inverse or not?
    If it passes the horizontal line test, than there exists an inverse.
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  11. #10 Re: inverse function 
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    Quote Originally Posted by Ellatha
    Quote Originally Posted by Heinsbergrelatz
    how do you tell if a specific function has an inverse or not?
    If it passes the horizontal line test, than there exists an inverse.
    That works if the function is a real-valued function of a real variable.
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  12. #11  
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    Quote Originally Posted by Guitarist
    Er, by showing it's a bijection. But one needs to know what the function IS.

    Look, this comes up from time-to-time here, and (I believe) confusion is caused by some horrid, but conventional, notation. So let's step through it.

    First, without any rigour, define an injection by: for any there is at most one such that .

    Now, with the same lack of rigour define a surjection as: for any there is at least one such that .

    Roll these together, and you have the bijection: for any there is at least one and at most one such that .

    Now. The "horrid" notation I referred to above arises as follows: no function, on sets or otherwise, is allowed to have multiple images (these being the result of applying our function to its domain, here ). But, by our surjection above, the preimage MUST be a non-empty set, say .

    The preimage of our injection is a set also. So, by my above, for the injection , then either or . This called a "singleton set"

    For the bijection we will therefore have that .

    In this particular circumstance is customary to ever so slightly abuse the notation, and to equate and by a further slight abuse to call the inverse of
    I read this, so you certainly didn't waste your time. Would you mind going through it with a little more rigor? The bijection, injection, surjection, and all the like? Notation and all if you could, I'm not well versed in the notation, but I can follow the explanations so far easily.
    Wise men speak because they have something to say; Fools, because they have to say something.
    -Plato

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  13. #12  
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    Quote Originally Posted by Arcane_Mathematician
    Quote Originally Posted by Guitarist
    Er, by showing it's a bijection. But one needs to know what the function IS.

    Look, this comes up from time-to-time here, and (I believe) confusion is caused by some horrid, but conventional, notation. So let's step through it.

    First, without any rigour, define an injection by: for any there is at most one such that .

    Now, with the same lack of rigour define a surjection as: for any there is at least one such that .

    Roll these together, and you have the bijection: for any there is at least one and at most one such that .

    Now. The "horrid" notation I referred to above arises as follows: no function, on sets or otherwise, is allowed to have multiple images (these being the result of applying our function to its domain, here ). But, by our surjection above, the preimage MUST be a non-empty set, say .

    The preimage of our injection is a set also. So, by my above, for the injection , then either or . This called a "singleton set"

    For the bijection we will therefore have that .

    In this particular circumstance is customary to ever so slightly abuse the notation, and to equate and by a further slight abuse to call the inverse of
    I read this, so you certainly didn't waste your time. Would you mind going through it with a little more rigor? The bijection, injection, surjection, and all the like? Notation and all if you could, I'm not well versed in the notation, but I can follow the explanations so far easily.
    injection ----- aka one-one

    surjection ----- aka onto

    bijection --- one-to-one and onto
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  14. #13  
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    And with two sets, say X and Y, a function will be an injection so long as there is, for every at most on such that . does that mean that there can be no x, such that ? is there a requirement that at least one exists such that for one ? Does that make any sense?
    Wise men speak because they have something to say; Fools, because they have to say something.
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  15. #14  
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    Quote Originally Posted by Arcane_Mathematician
    And with two sets, say X and Y, a function will be an injection so long as there is, for every at most on such that . does that mean that there can be no x, such that ? is there a requirement that at least one exists such that for one ? Does that make any sense?
    You have the definition of an injection (one-to-one function) correct.

    An injection need not be onto, so there may be a y that is not f(x) for any x.

    If every y is f(x) for some x then f is "onto" or "surjective" (same thing).

    If f is both an injection and a surjection then it is a bijection. A bijection and an invertible function are the same thing.
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  16. #15  
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    Quote Originally Posted by Arcane_Mathematician
    Would you mind going through it with a little more rigor? The bijection, injection, surjection, and all the like? Notation and all if you could, I'm not well versed in the notation, but I can follow the explanations so far easily.
    Well I am not sure I can add too much to my previous, except perhaps this.

    It essential to specify what sort of objects our function takes as input and what sort of objects it returns as output; these are known respectively as the domain and the codomain of our function.

    Take the simple case that , the integers, where, say, . This is obviously an injection, since 3 is obviously integer, yet there is no such that . One says that 3 is not in the range of our function.

    But if I choose another codomain, say the even numbers (those integers exactly divisible by 2), I may have (this is standard notation for the even integers) is a bijection; one half of every even integer is an integer, even or odd. But notice that , so we have that an injection from a set to a set can be a bijection from a set to a subset.

    Or consider another function with the same domain and codomain, say , This is clearly a surjection, as for any integer. But again, by changing the domain to the non-negative integers, call them we may recover a bijection.

    Again, a surjection from a set set to a set may be a bijection from a subset to a set. I hope you can see the symmetry here.

    In fact, if we have some reason (we often do) to retain our original domain, and by noting that we may have what is called the restriction of the function to this subset of the domain, which is usually written , which in the case in point is our bijection

    As far as I am aware this is not used all that often, but there are cases from trigonometry where it is very widely used. Can you think what they are?
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  17. #16  
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    the functions arc-sine and arc-cosine pop into mind immediately, where the domain is restricted only to one half of a cycle for each of the respective functions. I imagine it would also be used for the inverse of tangent, though less obviously.
    Wise men speak because they have something to say; Fools, because they have to say something.
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