Taylor series expansion of $frac{1}{sqrt{1-beta x(x+1)}}$












6












$begingroup$


I am trying to find the taylor series expansion about $0$ (maclaurin series) of $$x rightarrow frac{1}{sqrt{1-beta x(x+1)}} text{ with } beta in mathbb{R}^{+*}$$



I've tried using the taylor series expansion of $$frac{1}{sqrt{1-X}} = sum_{n=0}^{infty}4^{-n}{2n choose n}X^n text{ }text{ }text{ }text{ with } text{ } X=beta x(x+1)$$



But I can't turn it into a power series because of the $(x+1)^n$...



I've also tried to derive $$frac{1}{n!}cdotfrac{text{d}^n}{text{d}x^n}left(frac{1}{sqrt{1-beta x(x+1)}}right)_{x=0}$$
But no results so far...



Edit : with a more powerful method, I found that, if we call $left(a_nright)_{ninmathbb{N}}$ the coefficients of the taylor series expansion $left(frac{1}{sqrt{1-beta x(x+1)}}=sum_{n=0}^{infty}a_nx^nright)$, the sequence $left(a_nright)_{ninmathbb{N}}$ is then defined by :
$$forall ngeq3, text{} na_n=betaleft(n-frac{1}{2}right)a_{n-1}+betaleft(n-1right)a_{n-2}$$
$$text{with }text{ }a_1 = frac{beta}{2} text{ , } a_2 = frac{3}{8}beta^2+frac{1}{2}beta$$



It's definitely a step forward, but I don't know how to proceed from there. Is there a way to handle sequences that are defined by such a way ?










share|cite|improve this question











$endgroup$

















    6












    $begingroup$


    I am trying to find the taylor series expansion about $0$ (maclaurin series) of $$x rightarrow frac{1}{sqrt{1-beta x(x+1)}} text{ with } beta in mathbb{R}^{+*}$$



    I've tried using the taylor series expansion of $$frac{1}{sqrt{1-X}} = sum_{n=0}^{infty}4^{-n}{2n choose n}X^n text{ }text{ }text{ }text{ with } text{ } X=beta x(x+1)$$



    But I can't turn it into a power series because of the $(x+1)^n$...



    I've also tried to derive $$frac{1}{n!}cdotfrac{text{d}^n}{text{d}x^n}left(frac{1}{sqrt{1-beta x(x+1)}}right)_{x=0}$$
    But no results so far...



    Edit : with a more powerful method, I found that, if we call $left(a_nright)_{ninmathbb{N}}$ the coefficients of the taylor series expansion $left(frac{1}{sqrt{1-beta x(x+1)}}=sum_{n=0}^{infty}a_nx^nright)$, the sequence $left(a_nright)_{ninmathbb{N}}$ is then defined by :
    $$forall ngeq3, text{} na_n=betaleft(n-frac{1}{2}right)a_{n-1}+betaleft(n-1right)a_{n-2}$$
    $$text{with }text{ }a_1 = frac{beta}{2} text{ , } a_2 = frac{3}{8}beta^2+frac{1}{2}beta$$



    It's definitely a step forward, but I don't know how to proceed from there. Is there a way to handle sequences that are defined by such a way ?










    share|cite|improve this question











    $endgroup$















      6












      6








      6


      1



      $begingroup$


      I am trying to find the taylor series expansion about $0$ (maclaurin series) of $$x rightarrow frac{1}{sqrt{1-beta x(x+1)}} text{ with } beta in mathbb{R}^{+*}$$



      I've tried using the taylor series expansion of $$frac{1}{sqrt{1-X}} = sum_{n=0}^{infty}4^{-n}{2n choose n}X^n text{ }text{ }text{ }text{ with } text{ } X=beta x(x+1)$$



      But I can't turn it into a power series because of the $(x+1)^n$...



      I've also tried to derive $$frac{1}{n!}cdotfrac{text{d}^n}{text{d}x^n}left(frac{1}{sqrt{1-beta x(x+1)}}right)_{x=0}$$
      But no results so far...



      Edit : with a more powerful method, I found that, if we call $left(a_nright)_{ninmathbb{N}}$ the coefficients of the taylor series expansion $left(frac{1}{sqrt{1-beta x(x+1)}}=sum_{n=0}^{infty}a_nx^nright)$, the sequence $left(a_nright)_{ninmathbb{N}}$ is then defined by :
      $$forall ngeq3, text{} na_n=betaleft(n-frac{1}{2}right)a_{n-1}+betaleft(n-1right)a_{n-2}$$
      $$text{with }text{ }a_1 = frac{beta}{2} text{ , } a_2 = frac{3}{8}beta^2+frac{1}{2}beta$$



      It's definitely a step forward, but I don't know how to proceed from there. Is there a way to handle sequences that are defined by such a way ?










      share|cite|improve this question











      $endgroup$




      I am trying to find the taylor series expansion about $0$ (maclaurin series) of $$x rightarrow frac{1}{sqrt{1-beta x(x+1)}} text{ with } beta in mathbb{R}^{+*}$$



      I've tried using the taylor series expansion of $$frac{1}{sqrt{1-X}} = sum_{n=0}^{infty}4^{-n}{2n choose n}X^n text{ }text{ }text{ }text{ with } text{ } X=beta x(x+1)$$



      But I can't turn it into a power series because of the $(x+1)^n$...



      I've also tried to derive $$frac{1}{n!}cdotfrac{text{d}^n}{text{d}x^n}left(frac{1}{sqrt{1-beta x(x+1)}}right)_{x=0}$$
      But no results so far...



      Edit : with a more powerful method, I found that, if we call $left(a_nright)_{ninmathbb{N}}$ the coefficients of the taylor series expansion $left(frac{1}{sqrt{1-beta x(x+1)}}=sum_{n=0}^{infty}a_nx^nright)$, the sequence $left(a_nright)_{ninmathbb{N}}$ is then defined by :
      $$forall ngeq3, text{} na_n=betaleft(n-frac{1}{2}right)a_{n-1}+betaleft(n-1right)a_{n-2}$$
      $$text{with }text{ }a_1 = frac{beta}{2} text{ , } a_2 = frac{3}{8}beta^2+frac{1}{2}beta$$



      It's definitely a step forward, but I don't know how to proceed from there. Is there a way to handle sequences that are defined by such a way ?







      real-analysis calculus sequences-and-series power-series taylor-expansion






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Dec 14 '18 at 0:10







      Harmonic Sun

















      asked Dec 13 '18 at 22:27









      Harmonic SunHarmonic Sun

      71710




      71710






















          1 Answer
          1






          active

          oldest

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          1












          $begingroup$

          I would set $y = beta x(x+1)$. With that:
          begin{align}
          frac1{sqrt{1 - y}}
          &= sum_{n=0}^{infty} 4^{-n} binom{2n}{n} y^n\
          &= sum_{n=0}^{infty} 4^{-n} binom{2n}{n} beta^n x^n (x+1)^n\
          &= sum_{n=0}^{infty}sum_{k=0}^{n} 4^{-n} binom{2n}{n} binom{n}{k} beta^n x^n x^k \
          &= sum_{m=0}^{infty} a_m x^m
          end{align}

          where $m = n + k$, so $a_m = sum_{n=0}^{m} 4^{-n}binom{2n}{n}binom{n}{m-n} beta^n$.



          $a_m$ is simply an number which you can calculate doing some sums and products. For $m = 0, dots, 3$ the $a_m$ are $1,frac{beta }{2},frac{1}{8} beta (3 beta +4),frac{1}{16} beta ^2 (5
          beta +12)$
          .



          Mathematica "simplifies" $a_m$ to
          $$
          a_m = 4^{-m} binom{2 m}{m} beta ^m ,
          _2F_1left(frac{1}{2}-frac{m}{2},-frac{m}{2};frac{1}{2}-m;-frac{4}{beta
          }right)
          $$

          where $_2F_1$ denotes the Hypergeometric function.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Well actually, after numerical simulation, your numerical values don't seem to work...
            $endgroup$
            – Harmonic Sun
            Dec 14 '18 at 0:17












          • $begingroup$
            @HarmonicSun Yes, I had forgotten to include $beta$. The values are not as pretty now but you can still easily calculate them.
            $endgroup$
            – 0x539
            Dec 14 '18 at 0:20










          • $begingroup$
            Ok thanks a lot ! Additionaly, do you think that based on my edit on my initial question, one can derive a prettier closed form for the $a_m$ ?
            $endgroup$
            – Harmonic Sun
            Dec 14 '18 at 0:26










          • $begingroup$
            I might be picky but, in your formula for $a_m$, ${n choose m-n}$ is not defined for, say, n=0... Or do we take it a 0 conventionaly ?
            $endgroup$
            – Harmonic Sun
            Dec 14 '18 at 0:33










          • $begingroup$
            @HarmonicSun Yes, the Binomial coefficient $binom{a}{b}$ is understood to be zero when $a < b$ (and $a, b in mathbb{Z}$).
            $endgroup$
            – 0x539
            Dec 14 '18 at 0:39














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          1












          $begingroup$

          I would set $y = beta x(x+1)$. With that:
          begin{align}
          frac1{sqrt{1 - y}}
          &= sum_{n=0}^{infty} 4^{-n} binom{2n}{n} y^n\
          &= sum_{n=0}^{infty} 4^{-n} binom{2n}{n} beta^n x^n (x+1)^n\
          &= sum_{n=0}^{infty}sum_{k=0}^{n} 4^{-n} binom{2n}{n} binom{n}{k} beta^n x^n x^k \
          &= sum_{m=0}^{infty} a_m x^m
          end{align}

          where $m = n + k$, so $a_m = sum_{n=0}^{m} 4^{-n}binom{2n}{n}binom{n}{m-n} beta^n$.



          $a_m$ is simply an number which you can calculate doing some sums and products. For $m = 0, dots, 3$ the $a_m$ are $1,frac{beta }{2},frac{1}{8} beta (3 beta +4),frac{1}{16} beta ^2 (5
          beta +12)$
          .



          Mathematica "simplifies" $a_m$ to
          $$
          a_m = 4^{-m} binom{2 m}{m} beta ^m ,
          _2F_1left(frac{1}{2}-frac{m}{2},-frac{m}{2};frac{1}{2}-m;-frac{4}{beta
          }right)
          $$

          where $_2F_1$ denotes the Hypergeometric function.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Well actually, after numerical simulation, your numerical values don't seem to work...
            $endgroup$
            – Harmonic Sun
            Dec 14 '18 at 0:17












          • $begingroup$
            @HarmonicSun Yes, I had forgotten to include $beta$. The values are not as pretty now but you can still easily calculate them.
            $endgroup$
            – 0x539
            Dec 14 '18 at 0:20










          • $begingroup$
            Ok thanks a lot ! Additionaly, do you think that based on my edit on my initial question, one can derive a prettier closed form for the $a_m$ ?
            $endgroup$
            – Harmonic Sun
            Dec 14 '18 at 0:26










          • $begingroup$
            I might be picky but, in your formula for $a_m$, ${n choose m-n}$ is not defined for, say, n=0... Or do we take it a 0 conventionaly ?
            $endgroup$
            – Harmonic Sun
            Dec 14 '18 at 0:33










          • $begingroup$
            @HarmonicSun Yes, the Binomial coefficient $binom{a}{b}$ is understood to be zero when $a < b$ (and $a, b in mathbb{Z}$).
            $endgroup$
            – 0x539
            Dec 14 '18 at 0:39


















          1












          $begingroup$

          I would set $y = beta x(x+1)$. With that:
          begin{align}
          frac1{sqrt{1 - y}}
          &= sum_{n=0}^{infty} 4^{-n} binom{2n}{n} y^n\
          &= sum_{n=0}^{infty} 4^{-n} binom{2n}{n} beta^n x^n (x+1)^n\
          &= sum_{n=0}^{infty}sum_{k=0}^{n} 4^{-n} binom{2n}{n} binom{n}{k} beta^n x^n x^k \
          &= sum_{m=0}^{infty} a_m x^m
          end{align}

          where $m = n + k$, so $a_m = sum_{n=0}^{m} 4^{-n}binom{2n}{n}binom{n}{m-n} beta^n$.



          $a_m$ is simply an number which you can calculate doing some sums and products. For $m = 0, dots, 3$ the $a_m$ are $1,frac{beta }{2},frac{1}{8} beta (3 beta +4),frac{1}{16} beta ^2 (5
          beta +12)$
          .



          Mathematica "simplifies" $a_m$ to
          $$
          a_m = 4^{-m} binom{2 m}{m} beta ^m ,
          _2F_1left(frac{1}{2}-frac{m}{2},-frac{m}{2};frac{1}{2}-m;-frac{4}{beta
          }right)
          $$

          where $_2F_1$ denotes the Hypergeometric function.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Well actually, after numerical simulation, your numerical values don't seem to work...
            $endgroup$
            – Harmonic Sun
            Dec 14 '18 at 0:17












          • $begingroup$
            @HarmonicSun Yes, I had forgotten to include $beta$. The values are not as pretty now but you can still easily calculate them.
            $endgroup$
            – 0x539
            Dec 14 '18 at 0:20










          • $begingroup$
            Ok thanks a lot ! Additionaly, do you think that based on my edit on my initial question, one can derive a prettier closed form for the $a_m$ ?
            $endgroup$
            – Harmonic Sun
            Dec 14 '18 at 0:26










          • $begingroup$
            I might be picky but, in your formula for $a_m$, ${n choose m-n}$ is not defined for, say, n=0... Or do we take it a 0 conventionaly ?
            $endgroup$
            – Harmonic Sun
            Dec 14 '18 at 0:33










          • $begingroup$
            @HarmonicSun Yes, the Binomial coefficient $binom{a}{b}$ is understood to be zero when $a < b$ (and $a, b in mathbb{Z}$).
            $endgroup$
            – 0x539
            Dec 14 '18 at 0:39
















          1












          1








          1





          $begingroup$

          I would set $y = beta x(x+1)$. With that:
          begin{align}
          frac1{sqrt{1 - y}}
          &= sum_{n=0}^{infty} 4^{-n} binom{2n}{n} y^n\
          &= sum_{n=0}^{infty} 4^{-n} binom{2n}{n} beta^n x^n (x+1)^n\
          &= sum_{n=0}^{infty}sum_{k=0}^{n} 4^{-n} binom{2n}{n} binom{n}{k} beta^n x^n x^k \
          &= sum_{m=0}^{infty} a_m x^m
          end{align}

          where $m = n + k$, so $a_m = sum_{n=0}^{m} 4^{-n}binom{2n}{n}binom{n}{m-n} beta^n$.



          $a_m$ is simply an number which you can calculate doing some sums and products. For $m = 0, dots, 3$ the $a_m$ are $1,frac{beta }{2},frac{1}{8} beta (3 beta +4),frac{1}{16} beta ^2 (5
          beta +12)$
          .



          Mathematica "simplifies" $a_m$ to
          $$
          a_m = 4^{-m} binom{2 m}{m} beta ^m ,
          _2F_1left(frac{1}{2}-frac{m}{2},-frac{m}{2};frac{1}{2}-m;-frac{4}{beta
          }right)
          $$

          where $_2F_1$ denotes the Hypergeometric function.






          share|cite|improve this answer











          $endgroup$



          I would set $y = beta x(x+1)$. With that:
          begin{align}
          frac1{sqrt{1 - y}}
          &= sum_{n=0}^{infty} 4^{-n} binom{2n}{n} y^n\
          &= sum_{n=0}^{infty} 4^{-n} binom{2n}{n} beta^n x^n (x+1)^n\
          &= sum_{n=0}^{infty}sum_{k=0}^{n} 4^{-n} binom{2n}{n} binom{n}{k} beta^n x^n x^k \
          &= sum_{m=0}^{infty} a_m x^m
          end{align}

          where $m = n + k$, so $a_m = sum_{n=0}^{m} 4^{-n}binom{2n}{n}binom{n}{m-n} beta^n$.



          $a_m$ is simply an number which you can calculate doing some sums and products. For $m = 0, dots, 3$ the $a_m$ are $1,frac{beta }{2},frac{1}{8} beta (3 beta +4),frac{1}{16} beta ^2 (5
          beta +12)$
          .



          Mathematica "simplifies" $a_m$ to
          $$
          a_m = 4^{-m} binom{2 m}{m} beta ^m ,
          _2F_1left(frac{1}{2}-frac{m}{2},-frac{m}{2};frac{1}{2}-m;-frac{4}{beta
          }right)
          $$

          where $_2F_1$ denotes the Hypergeometric function.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Dec 14 '18 at 0:30

























          answered Dec 14 '18 at 0:03









          0x5390x539

          1,450518




          1,450518












          • $begingroup$
            Well actually, after numerical simulation, your numerical values don't seem to work...
            $endgroup$
            – Harmonic Sun
            Dec 14 '18 at 0:17












          • $begingroup$
            @HarmonicSun Yes, I had forgotten to include $beta$. The values are not as pretty now but you can still easily calculate them.
            $endgroup$
            – 0x539
            Dec 14 '18 at 0:20










          • $begingroup$
            Ok thanks a lot ! Additionaly, do you think that based on my edit on my initial question, one can derive a prettier closed form for the $a_m$ ?
            $endgroup$
            – Harmonic Sun
            Dec 14 '18 at 0:26










          • $begingroup$
            I might be picky but, in your formula for $a_m$, ${n choose m-n}$ is not defined for, say, n=0... Or do we take it a 0 conventionaly ?
            $endgroup$
            – Harmonic Sun
            Dec 14 '18 at 0:33










          • $begingroup$
            @HarmonicSun Yes, the Binomial coefficient $binom{a}{b}$ is understood to be zero when $a < b$ (and $a, b in mathbb{Z}$).
            $endgroup$
            – 0x539
            Dec 14 '18 at 0:39




















          • $begingroup$
            Well actually, after numerical simulation, your numerical values don't seem to work...
            $endgroup$
            – Harmonic Sun
            Dec 14 '18 at 0:17












          • $begingroup$
            @HarmonicSun Yes, I had forgotten to include $beta$. The values are not as pretty now but you can still easily calculate them.
            $endgroup$
            – 0x539
            Dec 14 '18 at 0:20










          • $begingroup$
            Ok thanks a lot ! Additionaly, do you think that based on my edit on my initial question, one can derive a prettier closed form for the $a_m$ ?
            $endgroup$
            – Harmonic Sun
            Dec 14 '18 at 0:26










          • $begingroup$
            I might be picky but, in your formula for $a_m$, ${n choose m-n}$ is not defined for, say, n=0... Or do we take it a 0 conventionaly ?
            $endgroup$
            – Harmonic Sun
            Dec 14 '18 at 0:33










          • $begingroup$
            @HarmonicSun Yes, the Binomial coefficient $binom{a}{b}$ is understood to be zero when $a < b$ (and $a, b in mathbb{Z}$).
            $endgroup$
            – 0x539
            Dec 14 '18 at 0:39


















          $begingroup$
          Well actually, after numerical simulation, your numerical values don't seem to work...
          $endgroup$
          – Harmonic Sun
          Dec 14 '18 at 0:17






          $begingroup$
          Well actually, after numerical simulation, your numerical values don't seem to work...
          $endgroup$
          – Harmonic Sun
          Dec 14 '18 at 0:17














          $begingroup$
          @HarmonicSun Yes, I had forgotten to include $beta$. The values are not as pretty now but you can still easily calculate them.
          $endgroup$
          – 0x539
          Dec 14 '18 at 0:20




          $begingroup$
          @HarmonicSun Yes, I had forgotten to include $beta$. The values are not as pretty now but you can still easily calculate them.
          $endgroup$
          – 0x539
          Dec 14 '18 at 0:20












          $begingroup$
          Ok thanks a lot ! Additionaly, do you think that based on my edit on my initial question, one can derive a prettier closed form for the $a_m$ ?
          $endgroup$
          – Harmonic Sun
          Dec 14 '18 at 0:26




          $begingroup$
          Ok thanks a lot ! Additionaly, do you think that based on my edit on my initial question, one can derive a prettier closed form for the $a_m$ ?
          $endgroup$
          – Harmonic Sun
          Dec 14 '18 at 0:26












          $begingroup$
          I might be picky but, in your formula for $a_m$, ${n choose m-n}$ is not defined for, say, n=0... Or do we take it a 0 conventionaly ?
          $endgroup$
          – Harmonic Sun
          Dec 14 '18 at 0:33




          $begingroup$
          I might be picky but, in your formula for $a_m$, ${n choose m-n}$ is not defined for, say, n=0... Or do we take it a 0 conventionaly ?
          $endgroup$
          – Harmonic Sun
          Dec 14 '18 at 0:33












          $begingroup$
          @HarmonicSun Yes, the Binomial coefficient $binom{a}{b}$ is understood to be zero when $a < b$ (and $a, b in mathbb{Z}$).
          $endgroup$
          – 0x539
          Dec 14 '18 at 0:39






          $begingroup$
          @HarmonicSun Yes, the Binomial coefficient $binom{a}{b}$ is understood to be zero when $a < b$ (and $a, b in mathbb{Z}$).
          $endgroup$
          – 0x539
          Dec 14 '18 at 0:39




















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