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Homework Help: Composite numbers and divisibility (2 problems)

  1. Sep 6, 2009 #1
    First of all, I hope this problem is supposed to be here - I'm Swedish and in Sweden "calculus" & "precalculus" are rather odd terms. Anyway..

    1. The problem statement, all variables and given/known data
    Prove that n3 - n is divisible by 6 if n is a natural number, and divisible by 24 if n is an odd natural number.

    2. Relevant equations

    3. The attempt at a solution
    There were two problems similar to this before this one and I attempted to solve it in the way I solved those (and the way the book solved them, as well), which was to split n into its even case and odd case.

    Even case:

    n = 2k

    (2k)3 - 2k = 8k3 - 2k , which obviously didn't help me much. But, as I'd used earlier for previous exercises, k can either be even or odd. If k is, say, even, k = 2s

    8(2s)3 - 2(2s) = 64s3 - 4s .. and I can continue on, but nothing is divisible by 6..

    And I've done the same for k = 2s + 1, which also doesn't work out. And I tried the odd case I mentioned earlier (n=2k +1) and that doesn't work out and I'm so close to screaming in frustration it's not even funny..

    1. The problem statement, all variables and given/known data
    Show that if n is a composite number it has a divisor greater than or equal to n½.

    2. Relevant equations

    3. The attempt at a solution
    My problem is that, yes, I absolutely and completely agree with the statement due to logic, but I've not a clue where to start the proof.. I'm not asking for someone to show me the proof, but a tip as to where to start?
    Last edited: Sep 6, 2009
  2. jcsd
  3. Sep 6, 2009 #2


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    Try proof by induction

    assume true for n=k

    Hk: k3-k=6A (A= integer)

    so Hk+1=(k+1)3-(k+1)

    expand and see if you can use k3-k=6A to show that Hk+1 is 6*something.
  4. Sep 6, 2009 #3
    I wanted to use mathematical induction, but we're not supposed to have learnt it yet (I just started a course at university, and we did induction at my previous school).

    And so, I tried MI, with the result: 6A + 3k2 + 3k .. divisible by 3, but not by 6.

    This problem is behaving very strangely, because at the same time I can plug in numbers and see that it is in fact true x/
  5. Sep 6, 2009 #4
    Make a substitution k=k3-6A, and see what you will come up with. :smile:

    6A+3(k3-6A)2+3(k3-6A) ?
  6. Sep 6, 2009 #5
    For the divisible by six, did you try just factoring the expression? Then your subbing in trick (2k+1) will work for the second half.

    For your second proof (that a composite number has a factor greater than [tex]\sqrt{n}[/tex], did you try contradiction?
  7. Sep 7, 2009 #6


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    Well, as pointed out by rock.freak667 in some earlier post, Proof of Induction can be used here.

    But, if you really want to solve the problem by splitting up into 2 cases, where n is even, and n is odd, you can try to factor. Like this:

    Notice that the final expression is already divisible by 2, so what's left is to prove that the rest is divisible by 3. And it's done. So,

    (2k)3 - 2k = 8k3 - 2k
    = 2(4k3 - k)
    = 2k(4k2 - 1)
    = 2k(2k - 1)(2k + 1)

    Look at the final expression, can you see why it's divisible by 6?

    - Is it divisible by 2? Why?
    - Is it divisible by 3? And why?


    And for the case where n is odd, you can tackle it in pretty much the same manner. Let's see if you still get suck. :)


    If n is a composite number, then there must exists some number a, and b, such that: n = ab, right? :)

    Now, let's do a little bit thinking, can both a, and b be less than n½?
    Last edited: Sep 7, 2009
  8. Sep 7, 2009 #7


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    I wouldn't use induction.

    [itex]n^3- n= n(n^2- 1)= (n-1)n(n+1)[/itex], the product of three consecutive numbers. You should show that, of any three consecutive numbers, at least one must be even and at least one must be a multiple of 3 so their product is a multiple of 6.

    If n is odd, then both n-1 and n+1 are even. Further, of two consecutive even numbers, 2k and 2k+2, one is a multiple of 4. (Look at the cases where k is even or odd.)
  9. Sep 7, 2009 #8
    Thanks for that last thing, that's exactly what I needed! :)

    And no, both a and b can't be less than n½, because there's no way it could become n. The thing is, I fully understand the logic, that either it's n½ * n½ or a>n½ * b<n½ (or the other way around, obviously).. but I can't get how to put it in mathematical language ..
  10. Sep 7, 2009 #9


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    Ok, that's what we call http://en.wikipedia.org/wiki/Proof_by_contradiction" [Broken], i.e, you are asked to prove some statement P. Now, you assume that you don't have P (i.e, P is false), and from there, try to find the contradiction.

    So, start from assuming that: "that composite number has a divisor greater than or equal to n½" is a false statement. Can you write out the negation of that statement? And then, what happens?
    Last edited by a moderator: May 4, 2017
  11. Sep 9, 2009 #10
    Oh yeah, I'd completely forgotten Proof by Contradicton.. hum. We never focused on it much.

    Anyway, I'd say something like:

    n=ab, a<n½, b<n½

    so, n < n½ * n½, but since it's the same base, n½ * n½ should be n1, and there's our contradiction.

    That's at least how I remember proof by contradiction..
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