1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Finding if M is Dense

  1. Aug 16, 2012 #1
    Hi, I am taking an intro. to elementary analysis class and so far our class has gone over sups/infs, the axiom stuff, archimedean property and now we are on dense sets. I've been stuck on this problem for a really long time trying to find clues on how to do it online. It goes as:

    Are the numbers of the form

    for m [itex]\in[/itex] N dense? What is the length of the largest interval that contains no such number?

    The book our class is using is Elementary Real Analysis 2nd ed. by bruckner/thomson. The book is online somewhere if needed. But anyways, I am really lost and a really helpful hint would be most appreciated. Thank you!
  2. jcsd
  3. Aug 16, 2012 #2
    What is the definition of "dense"?
  4. Aug 16, 2012 #3
    In my book, a set of E of R is said to be dense(or dense in R) if every interval (a,b) contains a point of E.
  5. Aug 16, 2012 #4
    Consider this interval: (0, 1/21000). Does it contain a point from your set?
  6. Aug 16, 2012 #5
    I am still fairly new with all this analysis stuff. For the interval, what made you choose it? I don't see how it would be in the set entirely. I feel like an idiot in this class.:cry:
  7. Aug 16, 2012 #6
    The interval that I chose or the interval (a, b) from the definition does not need to be in the set. Rather, it is the set must be such that whatever interval (a, b) is selected, there will be at least one point in that set that will also be in that interval. Your set consists of points that are separated by finite, albeit very small, distances from one another, so it is possible to select an interval - infinitely many intervals in fact - that will be smaller than the distance between any two points in your set, so they will not contain any point of the set. The set is not dense.

    A dense set in R is, for example, that of rational numbers, because in any given interval there are infinitely many of them.
  8. Aug 16, 2012 #7


    User Avatar
    Science Advisor

    You said "numbers of the form ±m/2100" vokD's number, 1/2100 is of that form with m= 1 so it certainly is in that set. In fact, it is the smallest positive number in that set. The next smaller number in the set is -1/2100 and the distance between them 1/2100- (-1/2100)= 2/2100= 1/299 and is the length of the largest interval of real numbers containing no member of the set. That much is not "analysis"- it's basic arithmetic.
  9. Aug 16, 2012 #8
    My number was 1/21000 :biggrin:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook