Looking for a proof for this counting problem












3














Consider all the coefficients in the expansion of
$$(x^a+x^b)^n$$
where $a,b,n$ are non negative integers.



Claim: The $color{Red}{textit{exponent}}$ with the $color{blue}{textit {maximum coefficient value}}$ in the expansion is $color{red}{lfloorfrac{n(a+b)}2rfloor}$

Example :
$(x^1+x^3)^4 = sumlimits_{k=0}^4binom{4}{k}x^{n-k}x^{3k} = x^{4} +4x^{6}+color{blue}{6}x^{color{red}{8}}+4x^{10}+x^{12}$



$color{red}{dfrac{4(1+3)}{2} = 8}$.



I feel this is a special case of central limit theorem in probability, but that theorem seems way more complicated to understand than my special case. So I'm trying to make sense of this result w/o using CLT. Any help ?










share|cite|improve this question




















  • 1




    Well you need to add something here, because this is false with $a=1$, $b=2$, and $n=1$.
    – BlarglFlarg
    Dec 2 '18 at 17:46










  • Oh sorry when n(a+b)/2 is not an integer, both the ceil and floor will have the same coefficients.. I'll add this in the question. Ty @BlarglFlarg
    – rsadhvika
    Dec 2 '18 at 17:49






  • 1




    It is just related to the binomial coefficients characteristics, nothing to do with a and b. Look at how they are calculated with Pascal' triangle
    – Damien
    Dec 2 '18 at 17:49






  • 1




    I know that the beauty of mathematics partially relies on the fact that it can connect seemingly unrelated topics but... how do you see a connection to CLT in this problem?
    – Taladris
    Dec 3 '18 at 0:31






  • 1




    @Taladris From what I understand it is (a bit) connected because $left(frac{x^a+x^b}{2}right)^n$ is the generating function of some sort of binomial distribution. (?)
    – Idéophage
    Dec 3 '18 at 4:47


















3














Consider all the coefficients in the expansion of
$$(x^a+x^b)^n$$
where $a,b,n$ are non negative integers.



Claim: The $color{Red}{textit{exponent}}$ with the $color{blue}{textit {maximum coefficient value}}$ in the expansion is $color{red}{lfloorfrac{n(a+b)}2rfloor}$

Example :
$(x^1+x^3)^4 = sumlimits_{k=0}^4binom{4}{k}x^{n-k}x^{3k} = x^{4} +4x^{6}+color{blue}{6}x^{color{red}{8}}+4x^{10}+x^{12}$



$color{red}{dfrac{4(1+3)}{2} = 8}$.



I feel this is a special case of central limit theorem in probability, but that theorem seems way more complicated to understand than my special case. So I'm trying to make sense of this result w/o using CLT. Any help ?










share|cite|improve this question




















  • 1




    Well you need to add something here, because this is false with $a=1$, $b=2$, and $n=1$.
    – BlarglFlarg
    Dec 2 '18 at 17:46










  • Oh sorry when n(a+b)/2 is not an integer, both the ceil and floor will have the same coefficients.. I'll add this in the question. Ty @BlarglFlarg
    – rsadhvika
    Dec 2 '18 at 17:49






  • 1




    It is just related to the binomial coefficients characteristics, nothing to do with a and b. Look at how they are calculated with Pascal' triangle
    – Damien
    Dec 2 '18 at 17:49






  • 1




    I know that the beauty of mathematics partially relies on the fact that it can connect seemingly unrelated topics but... how do you see a connection to CLT in this problem?
    – Taladris
    Dec 3 '18 at 0:31






  • 1




    @Taladris From what I understand it is (a bit) connected because $left(frac{x^a+x^b}{2}right)^n$ is the generating function of some sort of binomial distribution. (?)
    – Idéophage
    Dec 3 '18 at 4:47
















3












3








3







Consider all the coefficients in the expansion of
$$(x^a+x^b)^n$$
where $a,b,n$ are non negative integers.



Claim: The $color{Red}{textit{exponent}}$ with the $color{blue}{textit {maximum coefficient value}}$ in the expansion is $color{red}{lfloorfrac{n(a+b)}2rfloor}$

Example :
$(x^1+x^3)^4 = sumlimits_{k=0}^4binom{4}{k}x^{n-k}x^{3k} = x^{4} +4x^{6}+color{blue}{6}x^{color{red}{8}}+4x^{10}+x^{12}$



$color{red}{dfrac{4(1+3)}{2} = 8}$.



I feel this is a special case of central limit theorem in probability, but that theorem seems way more complicated to understand than my special case. So I'm trying to make sense of this result w/o using CLT. Any help ?










share|cite|improve this question















Consider all the coefficients in the expansion of
$$(x^a+x^b)^n$$
where $a,b,n$ are non negative integers.



Claim: The $color{Red}{textit{exponent}}$ with the $color{blue}{textit {maximum coefficient value}}$ in the expansion is $color{red}{lfloorfrac{n(a+b)}2rfloor}$

Example :
$(x^1+x^3)^4 = sumlimits_{k=0}^4binom{4}{k}x^{n-k}x^{3k} = x^{4} +4x^{6}+color{blue}{6}x^{color{red}{8}}+4x^{10}+x^{12}$



$color{red}{dfrac{4(1+3)}{2} = 8}$.



I feel this is a special case of central limit theorem in probability, but that theorem seems way more complicated to understand than my special case. So I'm trying to make sense of this result w/o using CLT. Any help ?







combinatorics






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Dec 2 '18 at 21:46









LutzL

56k42054




56k42054










asked Dec 2 '18 at 17:41









rsadhvika

1,6631228




1,6631228








  • 1




    Well you need to add something here, because this is false with $a=1$, $b=2$, and $n=1$.
    – BlarglFlarg
    Dec 2 '18 at 17:46










  • Oh sorry when n(a+b)/2 is not an integer, both the ceil and floor will have the same coefficients.. I'll add this in the question. Ty @BlarglFlarg
    – rsadhvika
    Dec 2 '18 at 17:49






  • 1




    It is just related to the binomial coefficients characteristics, nothing to do with a and b. Look at how they are calculated with Pascal' triangle
    – Damien
    Dec 2 '18 at 17:49






  • 1




    I know that the beauty of mathematics partially relies on the fact that it can connect seemingly unrelated topics but... how do you see a connection to CLT in this problem?
    – Taladris
    Dec 3 '18 at 0:31






  • 1




    @Taladris From what I understand it is (a bit) connected because $left(frac{x^a+x^b}{2}right)^n$ is the generating function of some sort of binomial distribution. (?)
    – Idéophage
    Dec 3 '18 at 4:47
















  • 1




    Well you need to add something here, because this is false with $a=1$, $b=2$, and $n=1$.
    – BlarglFlarg
    Dec 2 '18 at 17:46










  • Oh sorry when n(a+b)/2 is not an integer, both the ceil and floor will have the same coefficients.. I'll add this in the question. Ty @BlarglFlarg
    – rsadhvika
    Dec 2 '18 at 17:49






  • 1




    It is just related to the binomial coefficients characteristics, nothing to do with a and b. Look at how they are calculated with Pascal' triangle
    – Damien
    Dec 2 '18 at 17:49






  • 1




    I know that the beauty of mathematics partially relies on the fact that it can connect seemingly unrelated topics but... how do you see a connection to CLT in this problem?
    – Taladris
    Dec 3 '18 at 0:31






  • 1




    @Taladris From what I understand it is (a bit) connected because $left(frac{x^a+x^b}{2}right)^n$ is the generating function of some sort of binomial distribution. (?)
    – Idéophage
    Dec 3 '18 at 4:47










1




1




Well you need to add something here, because this is false with $a=1$, $b=2$, and $n=1$.
– BlarglFlarg
Dec 2 '18 at 17:46




Well you need to add something here, because this is false with $a=1$, $b=2$, and $n=1$.
– BlarglFlarg
Dec 2 '18 at 17:46












Oh sorry when n(a+b)/2 is not an integer, both the ceil and floor will have the same coefficients.. I'll add this in the question. Ty @BlarglFlarg
– rsadhvika
Dec 2 '18 at 17:49




Oh sorry when n(a+b)/2 is not an integer, both the ceil and floor will have the same coefficients.. I'll add this in the question. Ty @BlarglFlarg
– rsadhvika
Dec 2 '18 at 17:49




1




1




It is just related to the binomial coefficients characteristics, nothing to do with a and b. Look at how they are calculated with Pascal' triangle
– Damien
Dec 2 '18 at 17:49




It is just related to the binomial coefficients characteristics, nothing to do with a and b. Look at how they are calculated with Pascal' triangle
– Damien
Dec 2 '18 at 17:49




1




1




I know that the beauty of mathematics partially relies on the fact that it can connect seemingly unrelated topics but... how do you see a connection to CLT in this problem?
– Taladris
Dec 3 '18 at 0:31




I know that the beauty of mathematics partially relies on the fact that it can connect seemingly unrelated topics but... how do you see a connection to CLT in this problem?
– Taladris
Dec 3 '18 at 0:31




1




1




@Taladris From what I understand it is (a bit) connected because $left(frac{x^a+x^b}{2}right)^n$ is the generating function of some sort of binomial distribution. (?)
– Idéophage
Dec 3 '18 at 4:47






@Taladris From what I understand it is (a bit) connected because $left(frac{x^a+x^b}{2}right)^n$ is the generating function of some sort of binomial distribution. (?)
– Idéophage
Dec 3 '18 at 4:47












3 Answers
3






active

oldest

votes


















6














If you know the binomial expansion (which you seem to), then this follows almost immediately. The key point to remember is that the maximum value of the coefficient is obtained at $nchoose k$, where $k$ is the largest integer less than or equal to $frac{n}{2}$. To prove this, consider the ratios $frac{n choose k+1}{nchoose k}$ and see for what range this is greater than $1$. Plug in your value of $k$ and you get your result.



PS: Note that your exponents must actually appear in the expansion for this to happen.






share|cite|improve this answer























  • Ahh I kindof see now how this can be related to the max coefficient in binomial expansion. Ty :) Pretty sure you meant this : $$$$ To prove this, consider the ratios $frac{n choose k+1}{nchoose k}$ and see for what range this is $color{red}{greater ~than~ 1}$.
    – rsadhvika
    Dec 2 '18 at 18:01












  • Ah yes you’re right, I meant that the binomial coefficient itself is increasing, not the ratio. I’ll correct that.
    – Boshu
    Dec 2 '18 at 18:05



















5














$(x^a+x^b)^n=x^{na}(1+x^c)^n=sum_{k=0}^n{n choose k}x^{na+k(b-a)}$. Now, ${n choose k}$ is maximum for $k=[n/2].$ So, answer is $na+[n/2](b-a)$ which matches with your answer if $n$ is even.






share|cite|improve this answer





















  • $c = b-a$; I like your approach more. It is neat and to the point, but I started working on the other answer first so had to mark it best as it helped me first. Thank you so much John_Wick you're awesome :)
    – rsadhvika
    Dec 2 '18 at 21:46





















3














The exponents of the development run from $na$ to $nb$, in increments $b-a$. By symmetry, the requested exponent can only be



$$frac{n(a+b)}{2}$$ when the numerator is even, and



$$frac{n(a+b)pm(b-a)}{2}$$



otherwise (there will be two consecutive terms with equal coefficient).






share|cite|improve this answer























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    3 Answers
    3






    active

    oldest

    votes








    3 Answers
    3






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    6














    If you know the binomial expansion (which you seem to), then this follows almost immediately. The key point to remember is that the maximum value of the coefficient is obtained at $nchoose k$, where $k$ is the largest integer less than or equal to $frac{n}{2}$. To prove this, consider the ratios $frac{n choose k+1}{nchoose k}$ and see for what range this is greater than $1$. Plug in your value of $k$ and you get your result.



    PS: Note that your exponents must actually appear in the expansion for this to happen.






    share|cite|improve this answer























    • Ahh I kindof see now how this can be related to the max coefficient in binomial expansion. Ty :) Pretty sure you meant this : $$$$ To prove this, consider the ratios $frac{n choose k+1}{nchoose k}$ and see for what range this is $color{red}{greater ~than~ 1}$.
      – rsadhvika
      Dec 2 '18 at 18:01












    • Ah yes you’re right, I meant that the binomial coefficient itself is increasing, not the ratio. I’ll correct that.
      – Boshu
      Dec 2 '18 at 18:05
















    6














    If you know the binomial expansion (which you seem to), then this follows almost immediately. The key point to remember is that the maximum value of the coefficient is obtained at $nchoose k$, where $k$ is the largest integer less than or equal to $frac{n}{2}$. To prove this, consider the ratios $frac{n choose k+1}{nchoose k}$ and see for what range this is greater than $1$. Plug in your value of $k$ and you get your result.



    PS: Note that your exponents must actually appear in the expansion for this to happen.






    share|cite|improve this answer























    • Ahh I kindof see now how this can be related to the max coefficient in binomial expansion. Ty :) Pretty sure you meant this : $$$$ To prove this, consider the ratios $frac{n choose k+1}{nchoose k}$ and see for what range this is $color{red}{greater ~than~ 1}$.
      – rsadhvika
      Dec 2 '18 at 18:01












    • Ah yes you’re right, I meant that the binomial coefficient itself is increasing, not the ratio. I’ll correct that.
      – Boshu
      Dec 2 '18 at 18:05














    6












    6








    6






    If you know the binomial expansion (which you seem to), then this follows almost immediately. The key point to remember is that the maximum value of the coefficient is obtained at $nchoose k$, where $k$ is the largest integer less than or equal to $frac{n}{2}$. To prove this, consider the ratios $frac{n choose k+1}{nchoose k}$ and see for what range this is greater than $1$. Plug in your value of $k$ and you get your result.



    PS: Note that your exponents must actually appear in the expansion for this to happen.






    share|cite|improve this answer














    If you know the binomial expansion (which you seem to), then this follows almost immediately. The key point to remember is that the maximum value of the coefficient is obtained at $nchoose k$, where $k$ is the largest integer less than or equal to $frac{n}{2}$. To prove this, consider the ratios $frac{n choose k+1}{nchoose k}$ and see for what range this is greater than $1$. Plug in your value of $k$ and you get your result.



    PS: Note that your exponents must actually appear in the expansion for this to happen.







    share|cite|improve this answer














    share|cite|improve this answer



    share|cite|improve this answer








    edited Dec 2 '18 at 18:06

























    answered Dec 2 '18 at 17:47









    Boshu

    705315




    705315












    • Ahh I kindof see now how this can be related to the max coefficient in binomial expansion. Ty :) Pretty sure you meant this : $$$$ To prove this, consider the ratios $frac{n choose k+1}{nchoose k}$ and see for what range this is $color{red}{greater ~than~ 1}$.
      – rsadhvika
      Dec 2 '18 at 18:01












    • Ah yes you’re right, I meant that the binomial coefficient itself is increasing, not the ratio. I’ll correct that.
      – Boshu
      Dec 2 '18 at 18:05


















    • Ahh I kindof see now how this can be related to the max coefficient in binomial expansion. Ty :) Pretty sure you meant this : $$$$ To prove this, consider the ratios $frac{n choose k+1}{nchoose k}$ and see for what range this is $color{red}{greater ~than~ 1}$.
      – rsadhvika
      Dec 2 '18 at 18:01












    • Ah yes you’re right, I meant that the binomial coefficient itself is increasing, not the ratio. I’ll correct that.
      – Boshu
      Dec 2 '18 at 18:05
















    Ahh I kindof see now how this can be related to the max coefficient in binomial expansion. Ty :) Pretty sure you meant this : $$$$ To prove this, consider the ratios $frac{n choose k+1}{nchoose k}$ and see for what range this is $color{red}{greater ~than~ 1}$.
    – rsadhvika
    Dec 2 '18 at 18:01






    Ahh I kindof see now how this can be related to the max coefficient in binomial expansion. Ty :) Pretty sure you meant this : $$$$ To prove this, consider the ratios $frac{n choose k+1}{nchoose k}$ and see for what range this is $color{red}{greater ~than~ 1}$.
    – rsadhvika
    Dec 2 '18 at 18:01














    Ah yes you’re right, I meant that the binomial coefficient itself is increasing, not the ratio. I’ll correct that.
    – Boshu
    Dec 2 '18 at 18:05




    Ah yes you’re right, I meant that the binomial coefficient itself is increasing, not the ratio. I’ll correct that.
    – Boshu
    Dec 2 '18 at 18:05











    5














    $(x^a+x^b)^n=x^{na}(1+x^c)^n=sum_{k=0}^n{n choose k}x^{na+k(b-a)}$. Now, ${n choose k}$ is maximum for $k=[n/2].$ So, answer is $na+[n/2](b-a)$ which matches with your answer if $n$ is even.






    share|cite|improve this answer





















    • $c = b-a$; I like your approach more. It is neat and to the point, but I started working on the other answer first so had to mark it best as it helped me first. Thank you so much John_Wick you're awesome :)
      – rsadhvika
      Dec 2 '18 at 21:46


















    5














    $(x^a+x^b)^n=x^{na}(1+x^c)^n=sum_{k=0}^n{n choose k}x^{na+k(b-a)}$. Now, ${n choose k}$ is maximum for $k=[n/2].$ So, answer is $na+[n/2](b-a)$ which matches with your answer if $n$ is even.






    share|cite|improve this answer





















    • $c = b-a$; I like your approach more. It is neat and to the point, but I started working on the other answer first so had to mark it best as it helped me first. Thank you so much John_Wick you're awesome :)
      – rsadhvika
      Dec 2 '18 at 21:46
















    5












    5








    5






    $(x^a+x^b)^n=x^{na}(1+x^c)^n=sum_{k=0}^n{n choose k}x^{na+k(b-a)}$. Now, ${n choose k}$ is maximum for $k=[n/2].$ So, answer is $na+[n/2](b-a)$ which matches with your answer if $n$ is even.






    share|cite|improve this answer












    $(x^a+x^b)^n=x^{na}(1+x^c)^n=sum_{k=0}^n{n choose k}x^{na+k(b-a)}$. Now, ${n choose k}$ is maximum for $k=[n/2].$ So, answer is $na+[n/2](b-a)$ which matches with your answer if $n$ is even.







    share|cite|improve this answer












    share|cite|improve this answer



    share|cite|improve this answer










    answered Dec 2 '18 at 17:51









    John_Wick

    1,366111




    1,366111












    • $c = b-a$; I like your approach more. It is neat and to the point, but I started working on the other answer first so had to mark it best as it helped me first. Thank you so much John_Wick you're awesome :)
      – rsadhvika
      Dec 2 '18 at 21:46




















    • $c = b-a$; I like your approach more. It is neat and to the point, but I started working on the other answer first so had to mark it best as it helped me first. Thank you so much John_Wick you're awesome :)
      – rsadhvika
      Dec 2 '18 at 21:46


















    $c = b-a$; I like your approach more. It is neat and to the point, but I started working on the other answer first so had to mark it best as it helped me first. Thank you so much John_Wick you're awesome :)
    – rsadhvika
    Dec 2 '18 at 21:46






    $c = b-a$; I like your approach more. It is neat and to the point, but I started working on the other answer first so had to mark it best as it helped me first. Thank you so much John_Wick you're awesome :)
    – rsadhvika
    Dec 2 '18 at 21:46













    3














    The exponents of the development run from $na$ to $nb$, in increments $b-a$. By symmetry, the requested exponent can only be



    $$frac{n(a+b)}{2}$$ when the numerator is even, and



    $$frac{n(a+b)pm(b-a)}{2}$$



    otherwise (there will be two consecutive terms with equal coefficient).






    share|cite|improve this answer




























      3














      The exponents of the development run from $na$ to $nb$, in increments $b-a$. By symmetry, the requested exponent can only be



      $$frac{n(a+b)}{2}$$ when the numerator is even, and



      $$frac{n(a+b)pm(b-a)}{2}$$



      otherwise (there will be two consecutive terms with equal coefficient).






      share|cite|improve this answer


























        3












        3








        3






        The exponents of the development run from $na$ to $nb$, in increments $b-a$. By symmetry, the requested exponent can only be



        $$frac{n(a+b)}{2}$$ when the numerator is even, and



        $$frac{n(a+b)pm(b-a)}{2}$$



        otherwise (there will be two consecutive terms with equal coefficient).






        share|cite|improve this answer














        The exponents of the development run from $na$ to $nb$, in increments $b-a$. By symmetry, the requested exponent can only be



        $$frac{n(a+b)}{2}$$ when the numerator is even, and



        $$frac{n(a+b)pm(b-a)}{2}$$



        otherwise (there will be two consecutive terms with equal coefficient).







        share|cite|improve this answer














        share|cite|improve this answer



        share|cite|improve this answer








        edited Dec 2 '18 at 17:58

























        answered Dec 2 '18 at 17:51









        Yves Daoust

        124k671221




        124k671221






























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