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

Homework Help: Functions - Range vs Codomain

  1. Feb 9, 2007 #1

    danago

    User Avatar
    Gold Member

    Hi. This isnt directly a homework question, but it will help in general.

    Im having a little trouble understanding what the difference between a range and codomain is. For example, for the function [tex]f(x) = \frac{3}{{2x - 2}}[/tex], i understand that the domain is [tex]\{ x \in R:x \ne 1\}[/tex]. Now, i also believe that the possible values that can be outputted by the function is given by
    [tex]
    \{ f(x) \in R:f(x) \ne 0\}
    [/tex]. Is this the codomain or range?

    Thanks in advance,
    Dan.
     
  2. jcsd
  3. Feb 9, 2007 #2

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    The range/codomain is the image of the domain through the function

    [tex] \mbox{Ran}(f(x)):=\left\{ f(x)\left|\right x\in D(f(x)) \right\} [/tex]

    In your case, first make a plot of the function first.
     
  4. Feb 9, 2007 #3
    that is the range of the function. codomain is usually a superset (sometimes equal as well) of the range. its generally defined in the question itself, like f:R-->R (here both domain and codomain are the set of real nos.),but range will be a subset(or an equal set) of R depending upon the function definition.
     
  5. Feb 9, 2007 #4

    HallsofIvy

    User Avatar
    Science Advisor

    According to Wikipedia, the "codomain" of a function f:X-> Y is the set Y. The "range" is the subset of Y that f actually maps something onto.

    For example, if f:R->R is defined by f(x)= ex, then the "codomain" is R but the "range" is the set, R+, of all positive real numbers.

    Notice that you cannot tell the "codomain" of a function just from its "formula". I could just as easily define f:R->R+, with f(x)= ex. Now the codomain and domain would be the same.
     
  6. Feb 21, 2011 #5
    I have a doubt, I think we also cannot tell what the "domain" is just from the "formula" . We can say what it "is not" but we cant say what it "is".

    for instance we can define a function as f:[1,2]->R , with f(x) = ex . here "domain" is what we define(i.e [1,2]) ,"co-domain" is what we define(i.e R) , but "range" is obtained from the formula, which in this case would be [e,e^2]

    but the formula definitely can tell us what domain is not.
    ex :- f(x) = [tex]\sqrt{x}[/tex] we can't say domain is R. we have to define domain as R[tex]^{+}[/tex] or it's subsets.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook