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Binomial Theorem?

  1. Dec 25, 2008 #1
    Rudin's proof of lim n-> inf (p^(1/n)) = 1

    1+n*x_n <= (1 + x_n)^n = o

    I don't see it from the binomial theorem, which is what he says that is from.

    He also does things with the binomial theorem like:

    (1+x_n)^n >= ((n(n-1)) / 2) *x_n^2

    I'm not sure what he did to get these two inequalities.
  2. jcsd
  3. Dec 25, 2008 #2


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    Those are terms in the expansion given by the binomial theorem. Since there are more terms in the actual expansion, it's an inequality.
  4. Dec 26, 2008 #3


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    1+ n*x_n <= (1+ x_n)^n- o perhaps, not "= o". (1+ x_n)^n, by the binomial theorem that you mention in your title, says that (1+ x_n)^n= 1+ n x_n+ terms of higher order in x_n which will go to 0 as x_n goes to 0: o(x_n) or "small o". Assuming that x_n is positive, all those missing terms in the binomial expansion are positive and so the first two terms are less than or equal to the whole thing.

    n(n-1)/2 is the second binomial coefficient nC2= n!(2!(n-2)!)= n(n-1)/2. (1+ x_n)^n= 1+ n x_n+ (n(n-2)/2) x_n^2+ higher terms. Again, assuming that x_n is positive, the 'whole thing' is greater than or equal to just one term.
    Last edited by a moderator: Dec 26, 2008
  5. Dec 26, 2008 #4
    Thanks. I feel pretty stupid lol, but its been a while since ive seen that thing.
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