If you are solving a problem and you encounter a generating function that you haven't seen before, is there a way to derive its underlying sequence representation?

For example I came across $\frac{1}{1+x}$ and had to look it up to see that it had coefficients $(-1)^n$, but what if I encounter something that doesn't have a lookup somewhere? Is there a way I can actually generate the coefficient representation from the generating function without needing to resort to internet lookups?

  • $\begingroup$ I think you mean sequence, not series. $\endgroup$ – Thomas Andrews Jan 9 '16 at 3:09
  • $\begingroup$ @ThomasAndrews What's the difference? $\endgroup$ – Sean Hill Jan 9 '16 at 3:13
  • $\begingroup$ A series is a sequence of the form $x_1,x_1+x_2,x_1+x_2+x_3,\dots$. If you are looking for the coefficients of a generating function, you are seeking a sequence which is not (in general) a series. $\endgroup$ – Thomas Andrews Jan 9 '16 at 3:15
  • $\begingroup$ @ThomasAndrews Oh ok, then yes I am looking for the sequence $\endgroup$ – Sean Hill Jan 9 '16 at 3:35
  • $\begingroup$ for what sequence do you want a generating function? $\endgroup$ – janmarqz Jan 9 '16 at 3:57

This math.SE question has a good example of the technique when you can decompose your function into partial fractions.

Edit: Using the example from your comment, $\dfrac{1-x+x^2}{1+3x-x^2} = -1-\dfrac{2x+2}{x^2-3x-1}$.

Solving $\dfrac{2x+2}{x^2-3x-1} = \dfrac{A}{x-\alpha}+\dfrac{B}{x-\beta}$ where $\alpha,\beta = \dfrac{3\pm\sqrt{13}}{2}$ yields $A=\dfrac{2\alpha+2}{\alpha-\beta},B=\dfrac{2\beta+2}{\beta-\alpha}$. Then $$-1-\dfrac{A}{x-\alpha}-\dfrac{B}{x-\beta} = -1+\dfrac{A}{\alpha}\dfrac{1}{1-\frac{1}{\alpha}x}+\dfrac{B}{\beta}\dfrac{1}{1-\frac{1}{\beta}x}=-1+\dfrac{A}{\alpha}\sum_{n=0}^\infty\frac{1}{\alpha^n}x^n+\dfrac{B}{\beta}\sum_{n=0}^\infty\frac{1}{\beta^n}x^n$$ $$=-1+\sum_{n=0}^\infty\left(\frac{A}{\alpha^{n+1}}+\frac{B}{\beta^{n+1}}\right)x^n$$ so the sequence is $a_0=1, a_{n\ge1} = \dfrac{A}{\alpha^{n+1}}+\dfrac{B}{\beta^{n+1}}=\dfrac{2^{n+1}}{\sqrt{13}}\left[\dfrac{5+\sqrt{13}}{(3+\sqrt{13})^{n+1}}-\dfrac{5-\sqrt{13}}{(3-\sqrt{13})^{n+1}}\right]$, which looks like it came from some fibonacci-like recurrence.


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