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Principle of Induction

  1. Mar 3, 2009 #1
    1. The problem statement, all variables and given/known data

    Prove that for each [tex]n \in N[/tex] (aka natural numbers), [tex]2^n \geq n+1[/tex]

    2. Relevant equations

    3. The attempt at a solution

    Let the proposition P(n) be "[tex]2^n \geq n+1[/tex]"

    Clearly P(n) is true for n=1, [tex]2^1 \geq 1+1[/tex].

    We suppose P(k) is true, i.e., supposing that [tex]2^k \geq k+1[/tex] is true, then,

    [tex]2^{k+1} \geq (k+1)+1[/tex]

    I think it can then be rewritten as [tex]2.2^{k} \geq 2.(k+1)[/tex]. Does anyone know the next step? I'm not sure what to do from here...

  2. jcsd
  3. Mar 4, 2009 #2
    well, your induction hypothesis is:

    [tex]2^k\geq k+1[/tex]


    [tex]2^{k+1}=2*2^k\geq 2(k+1)=2k+2>(k+1)+1[/tex]
  4. Mar 4, 2009 #3


    Staff: Mentor

    Omit the line above, since that's where you want to end up. Just say:
    suppose P(k) is true, i.e., that [tex]2^k \geq k+1[/tex]
    Keep it in the back of your mind where you want to go, but work toward that goal.
    So you have 2k + 1 = 2 * 2k >= 2 * (k + 1) = 2k + 2

    How does that last expression compare with k + 2? (I.e., (k + 1) + 1)
  5. Mar 7, 2009 #4
    So, it is [tex]2^{k+1} = 2.2^k \geq 2.(k+1)[/tex]

    [tex]2(k+1) = (k+1)+(k+1) \geq (k+1)+1[/tex]

    I'm a little confused, how should we make the conclusion?
    Since (k+1)+1 is supposed to be less than or equal to [tex]2^{k+1} (= 2.2^k)[/tex], and now we know it is less than or equal to 2.(k+1). Therefore...
    Last edited: Mar 7, 2009
  6. Mar 8, 2009 #5


    User Avatar
    Science Advisor

    No, "(k+1)+(k+1)" is not the point. As Mark44 said before, 2(k+1)= 2k+ 2. Now, since k is a positive integer, it is certainly true that 2k> k (you might want to do another induction to prove that formally) so 2k+ 2> k+ 2= (k+1)+ 1.

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