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Sketch Domain of F(x,y)

  1. Oct 4, 2016 #1
    1. The problem statement, all variables and given/known data
    Suppose we have f(x,y) = ∑(x/y)^n , n goes from 0 to infinity. What is the domain on the xy plane? Sketch it.

    3. Attempt
    I was thinking to look at the scenario if x/y is less than 1, or bigger than 1. If the ratio is less than 1, then I can use an idea from geometric series to write out an explicit form for f(x,y) in which it will be defined for x>y, or in other words, below the line y = x. What about if x/y is bigger than 1? I am getting stuck here as how to represent that. Also I am not certain if the approach is even correct.
  2. jcsd
  3. Oct 4, 2016 #2


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    Homework Helper

    The approach is almost correct. Both x and y can be negative, and you can use the sum of geometric series if |x/y|<1. Does the sum exist in the opposite case? (Is it finite?)
  4. Oct 4, 2016 #3
    Oh youre right! I keep forgetting we look at the absolute value of the ratio.

    The sum does not exist in the opposite case, therefore any such combination where the ratio is greater than 1 is a domain violation and should be excluded from the sketch?
  5. Oct 4, 2016 #4


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    Yes.And do not forget y=0, when the ratio does not exist.
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