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Homework Help: Combinatorics: 1= ((n+k)Cr)*((1/2)^(n+k)) from k = 0 to n

  1. Apr 6, 2014 #1


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    (This is not HW, even though it may look a bit like it.)
    Using the notation nCr in the combinatorics meaning,
    The sum of ((n+k)Cn)*((1/2)^(n+k)) from k = 0 to n equals one. Why?
    (I thought it might use the identity (n+1)Cr = nCr+nC(r-1), but that didn't get me anywhere.)
    Thanks in advance for any pointers.
    Last edited: Apr 6, 2014
  2. jcsd
  3. Apr 6, 2014 #2

    Stephen Tashi

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  4. Apr 6, 2014 #3


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    :redface: oops, right. Thanks, Stephen Tashi, for spotting the typo. I have edited it.
    So, back to the question......
  5. Apr 6, 2014 #4
    Not sure you've stated it correctly, still. Is this what you wanted?

    [itex]1 = 1^n = (\frac{1}{2} + \frac{1}{2} )^n [/itex]

    [itex]= \Sigma_{k = 0}^n {{n}\choose{k}} \frac{1}{2^n}[/itex] (binomial expansion)
  6. Apr 6, 2014 #5


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    Thanks, homeomorphic, but no: my title stated it incorrectly (I forgot to edit the title, and now it no longer gives me that option), but I have edited the original question, as Stephen Tashi pointed out. The binomial expansion you suggest only gives me
    Sum of (nCk)*((1/2)^(k)) from k = 0 to n
    Note the differences to
    Sum of ((n+k)Cn)*((1/2)^(n+k)) from k = 0 to n
    As a random example, I asked Wolfram Alpha to compute this, with n=7, and it was indeed one. This is not a proof, but a reassurance that I wrote it right.
  7. Apr 6, 2014 #6
    And if it looks like homework, then it belongs in the homework forums! I'll move it for you :smile:
  8. Apr 6, 2014 #7


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    OK, thanks. Hope to get a response there. As a side note: the question arose as I was looking at Banach's matchbox problem.
  9. Apr 7, 2014 #8


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    reformulated:sum from k=0 to n of (n+k)Ck = 2^n

    Or, to put it another way, why is the sum from k=0 to n of ([(n+k)Ck]/2k) equal to 2n?
    Last edited: Apr 7, 2014
  10. Apr 7, 2014 #9
    I'm confused what the problem actually is. Is this the problem:

    [tex]\sum_{k=0}^n \binom{n+k}{k} \frac{1}{2^k} = 2^n[/tex]

    because the OP and the title both say completely different things.
  11. Apr 7, 2014 #10
    Or is it

    [tex]\sum_{k=0}^n \binom{n+k}{n} \frac{1}{2^k} = 2^n [/tex]
  12. Apr 7, 2014 #11


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    They are the same thing, since (n+k)Ck = (n+k)C[(n+k)-n]=(n+k)Cn.
    I merely restated it in the two fashions, because perhaps one formulation is easier to prove than the other.
  13. Apr 7, 2014 #12
    Oh right, haha. Shame on me for missing that... I'm gonna think about this for a while. I assume you have already tried induction?
  14. Apr 7, 2014 #13


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    Thanks for considering it, micromass. Of course I tried induction, but got stuck. This does not mean that induction is not the right way to go; it could very well just mean that I am missing an obvious step in the induction. In the induction step I get
    sum from k=0 to n of ([(n+k)C(k)]/2k) = 2n
    sum from k=0 to n+1 of ([(n+k+1)C(k)]/2k) = 2n+1
    sum from k=0 to n+1 of ([(n+k+1)C(k)]/2k) = [sum from k=0 to n of ([(n+k+1)Ck]/2k)]+[(2n+1)C(n+1)/2n+1] and then?
    (similarly with the other formulation)
  15. Apr 7, 2014 #14


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    Thanks very much, micromass! That's a path I had not considered. Very elegant.:cool:
  16. Apr 7, 2014 #15
    Yeah, it's also not entirely correct. You need to do something with the Taylor series

    [tex]\frac{y^n}{(1-y)^{n+1}} = \sum_{k=n}^{+\infty} \binom{k}{n} x^k[/tex]

    But then you also need some formula for

    [tex]\sum_{k=2n}^{+\infty} \binom{k}{n}x^k[/tex]

    which I haven't found yet.
  17. Apr 7, 2014 #16


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    Thanks again, micromass; at least for the moment it gives me another line of thought to pursue. Although this is in the "HW & HW-similar" section, it is not HW and so there is no "due date", so if you come up with something anytime in the future, I would be grateful to hear about it.
  18. Apr 7, 2014 #17
    OK, a highly nontrivial way is this:

    We are interested in

    [tex]x_a = \sum_{k=a}^{+\infty} \binom{k}{n} \frac{1}{2^k}[/tex]

    If we found this, then your sum is simply ##x_{n} - x_{2n+1}##. Now, it is easily seen that

    [tex]x_a = \binom{a}{n} \frac{1}{2^a} {}_2F_1(1,a+1;a-n+1;1/2)[/tex]

    Where ##{}_2F_1## is the hypergeometric function (it's really easy to see, it just follows from the definition, I might have made some computation errors tho). So we want to calculate


    The former can be calculated by applying a Euler transformation and yields ##2^{n+1}##: http://en.wikipedia.org/wiki/Hypergeometric_function#Fractional_linear_transformations

    The first one, I can't find at the moment.
  19. Apr 7, 2014 #18


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    Thanks, micromass. As you wrote, non-trivial, but it gives me some new techniques to get acquainted with.
  20. Apr 7, 2014 #19
    The second one [tex]{}_2F_1(1,2n+2;n+3;1/2)[/tex] can be calculated by applying Gauss' contiguous relations. Specifically, I'm applying

    [tex]\frac{(c-a){}_2F_1(a-1,b;c;z) + (a -c + bz){}_2F_1(a,b;c;z)}{1-z} = (c-1)({}_2F_1(a,b;c-1;z) - {}_2F_1(a,b;c;z))[/tex]

    This means that we simply need to find ##{}_2F_1(0,2n+2;n+3;1/2)## which is easily done by applying the definition. We also need to find ##{}_2F_1(1,2n+2;n+2;1/2)##. But this can be done by Gauss' second summation theorem: http://en.wikipedia.org/wiki/Hypergeometric_function#Values_at_z.C2.A0.3D.C2.A01.2F2
  21. Apr 7, 2014 #20
    Well, this works, but an elementary approach still eludes me.
  22. Apr 7, 2014 #21


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  23. Apr 7, 2014 #22
    OK, but that solves it right? I don't get why you say "special case"? I'll write it here for future reference:

    You need to find

    [tex]\sum_{k=0}^n \binom{n+k}{n} \frac{1}{2^{n+k}}[/tex]

    Multiply by ##4^n/4^n## to get

    [tex]\frac{1}{4^n} \sum_{k=0}^n \binom{n+k}{n} 2^{n-k}[/tex]

    Apply the relation ##2^a = \sum_{j=0}^a \binom{a}{j}## to get

    [tex]\frac{1}{4^n} \sum_{k=0}^n \binom{n+k}{n} \sum_{j=0}^{n-k} \binom{n-k}{j}[/tex]

    Switching around the sums and allowing the convention that if ##\binom{a}{b}=0## if ##b>a##, we get

    [tex]\frac{1}{4^n} \sum_{j=0}^n \sum_{k=0}^n \binom{n-k}{j}\binom{n+k}{n}[/tex]

    the relation in Knuth is

    [tex]\sum_{k=0}^l \binom{l-k}{m}\binom{q+k}{n} = \binom{l+q+1}{m+n+1}[/tex]

    Thus we get

    [tex]\frac{1}{4^n} \sum_{j=0}^n \binom{2n+1}{j+n+1}[/tex]

    Change variables ##l=j+n+1##:

    [tex]\frac{1}{4^n} \sum_{l=n+1}^{2n+1} \binom{2n+1}{l}[/tex]

    Applying that ##\binom{2n+1}{l} = \binom{2n+1}{2n+1-l}##, we see that

    [tex]\frac{1}{2^{2n-1}} \sum_{l=0}^{2n+1} \binom{2n+1}{l}[/tex]

    Again applying ##2^a = \sum_{j=0}^a \binom{a}{j}##, we see that this is equal to

    [tex]\frac{1}{2^{2n+1}}2^{2n+1} = 1[/tex]
    Last edited: Apr 7, 2014
  24. Apr 7, 2014 #23


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    Thanks again, micromass. Yes, the phrase "a special case" was misplaced; I was being overly careful before I had seen the reference in Knuth et al. (I have in fact not yet obtained a copy, so I still need to look at it.) Apparently, as you say, this now solves it, and your exposition is nice and clear -- except that I think you left something out in the line
    "Applying that ##\binom{2n+1}{l} = \binom{2n+1}{l}##, we see that..."
    Last edited: Apr 7, 2014
  25. Apr 7, 2014 #24
    Thank you, that was a typo. Well, all that remains is proving the relation in Knuth, which is not done there. I think an induction proof works, but I don't like that. Consider the function

    [tex]f_n(x) = \frac{x^n}{(1-x)^{n+1}}[/tex]

    We know the binomial series

    [tex]\frac{1}{(1-x)^{n+1}} = \sum_{k=0}^{+\infty} \binom{k+n}{n} x^k[/tex]


    [tex]f_n(x) = \sum_{k=0}^{+\infty} \binom{k}{n} x^k[/tex]

    with the usual convention that ##\binom{k}{n} = 0## if ##n>k##

    Thus ##xf_n(x)f_m(x) = f_{n+m+1}(x)##. By the Cauchy product of series, we then see that

    [tex]\sum_{k=-1}^{+\infty} \binom{k+1}{n+m+1} x^k = \sum_{k=0}^{+\infty} x^k \sum_{j=0}^k \binom{j}{n}\binom{k-j}{m}[/tex]

    Thus we see the identity (with additionally substituting ##t=k##

    [tex]\sum_{j=0}^k \binom{j}{n}\binom{t-j}{m} = \binom{t+1}{n+m+1}[/tex]

    Substituting ##j=l-k##, we get

    [tex]\sum_{k=0}^{l-k} \binom{l-k}{n}\binom{t-l+k}{m} = \binom{t+1}{n+m+1}[/tex]

    The result now follows after substiting ##q=t-l##.
  26. Apr 7, 2014 #25


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    Excellent! Thanks so very much. Yes, I finally got hold of Knuth and was surprised not to find a derivation for the the principle in question. So your explanation definitely fills a gap, completing the answer to my question. I am very grateful.
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