# What is the sum of the infinite series $\sum_{n=1}^{\infty}\frac{(-1)^{n}}{4n^{2}-1}$?

Could someone please help me with what is the sum of the $$\sum_{n=1}^{\infty}\frac{(-1)^{n}}{4n^{2}-1}$$ infinite series? You can find the sum of $$\sum_{n=1}^{\infty}\frac{1}{4n^{2}-1}$$ here. Thank you.

hint Per the other one, it suffices to compute $$-\sum_{n=1}^\infty (-1)^n\left(\frac{1}{2n-1} - \frac{1}{2n+1}\right),$$ (and then multiply by $-1/2$) but now we lose the nice telescoping property and instead have $$\left(1 - \frac{1}{3}\right) -\left(\frac{1}{3} - \frac{1}{5}\right) +\left(\frac{1}{5} - \frac{1}{7}\right)-\ldots = 1 -\frac{2}{3}+\frac{2}{5} - \frac{2}{7}\ldots.$$ But the sum $$1-\frac{1}{3} +\frac{1}{5}-\frac{1}{7}+\ldots$$ is well known and closely related.

edit

We have $$1-\frac{2}{3} + \frac{2}{5} -\frac{2}{7} = -1 + 2\left(1-\frac{1}{3} + \frac{1}{5}-\ldots \right) = -1+\frac{\pi}{2}.$$ Because the series I opted to start with is your series times $-2,$ the final answer is $-1/2$ times this: $\pi/4-2.$

The formula $$1-\frac{1}{3} + \frac{1}{5}-\ldots = \frac{\pi}{4}$$ follows from the Taylor series $$\arctan(x) = x-\frac{1}{3}x^3+\frac{1}{5}x^5 -\ldots$$ evaluated at $x=1.$ So my answer much different in substance from Olivier's.

• Wait... isn’t that equal to $\dfrac \pi 4$ ?? I could be wrong, because after $\dfrac 17$ there should be a $+$ operation instead of $-$ since it is a constant alternating sum. – Mr Pie Jan 17 '18 at 1:57
• @user477343 The last sum I wrote down is $\pi/4,$ from which it follows that OP's sum is $1/2-\pi/4.$ (And you're right that I had a typo with the last sign.) – spaceisdarkgreen Jan 17 '18 at 2:06
• Could you please extend your answer with more details? It would be very nice of you. By the way why is it well known that $\sum_{n=1}^{\infty}\frac{(-1)^{n+1}}{2n+1}=\frac{\pi}{4}$? – bencemeszaros Jan 17 '18 at 2:26
• @bencemeszaros Well that's Leibniz's formula, isn't it? Discovered in $1673$ CE I believe... although it was discovered earlier $\approx 1400$ CE..... Oh wait, I though you were asking me XD – Mr Pie Jan 17 '18 at 2:50
• @bencemeszaros k updated. User gave the history lesson. I believe it was first introduced and motivated to me as a horrendously slow and inefficient way to compute $\pi.$ – spaceisdarkgreen Jan 17 '18 at 3:22

Assume $|x|<1$. One may recall that $$\sum_{n=1}^\infty\frac{(-1)^{n-1}x^{2n-1}}{2n-1}=\arctan x$$then writing \begin{align} 2\sum_{n=1}^\infty\frac{(-1)^{n}x^{2n-1}}{4n^{2}-1}&=\sum_{n=1}^\infty\frac{(-1)^{n}x^{2n-1}}{2n-1}-\sum_{n=1}^\infty\frac{(-1)^{n}x^{2n-1}}{2n+1} \end{align} gives

\begin{align} \sum_{n=1}^\infty\frac{(-1)^{n}x^{2n-1}}{4n^{2}-1}&=\frac1{2x}-\frac12\left(\frac1{x^2}+1\right)\cdot\arctan x,\qquad 0<|x|<1. \end{align}

In particular, by letting $x \to 1^-$, one finds

$$\sum_{n=1}^\infty\frac{(-1)^{n}}{4n^{2}-1}=\frac12-\frac \pi4$$

using $\displaystyle \arctan 1= \frac \pi4$.

• Thank you for the answer. I accepted the other one because it was more intuitive for me. – bencemeszaros Jan 17 '18 at 10:56

Or using our favourite alternative of converting the sum into a double integral.

As $$\sum_{n = 1}^\infty \frac{(-1)^n}{4n^2 - 1} = \sum_{n = 1}^\infty \frac{(-1)^n}{(2n - 1)(2n + 1)},$$ noting that $$\frac{1}{2n - 1} = \int_0^1 x^{2n - 2} \, dx \qquad \text{and} \qquad \frac{1}{2n + 1} = \int_0^1 y^{2n} \, dy,$$ the sum can be rewritten as \begin{align*} \sum_{n = 1}^\infty \frac{(-1)^n}{4n^2 - 1} &= \sum_{n = 1}^\infty (-1)^n \int_0^1 \int_0^1 x^{2n - 2} y^{2n} \, dx dy\\ &= \int_0^1 \int_0^1 \sum_{n = 1}^\infty (-1)^n x^{2n - 2} y^{2n} \,dx dy \tag1\\ &= \int_0^1 \int_0^1 \frac{1}{x^2} \sum_{n = 1}^\infty (-x^2 y^2)^n \, dx dy\\ &= \int_0^1 \int_0^1 \frac{1}{x^2} \cdot \frac{-x^2 y^2}{1 + x^2 y^2} \, dx dy \tag2\\ &= - \int_0^1 \int_0^1 \frac{y^2}{1 + x^2 y^2} \, dx dy\\ &= - \int_0^1 \Big{[} y \tan^{-1} (xy) \Big{]}_0^1 \, dy\\ &= -\int_0^1 y \tan^{-1} y \, dy\\ &= -\left [\frac{y^2}{2} \tan^{-1} y \right ]_0^1 + \frac{1}{2} \int_0^1 \frac{y^2}{1 + y^2} \, dy \tag3\\ &= -\frac{\pi}{8} + \frac{1}{2} \int_0^1 \frac{(1 + y^2) - 1}{1 + y^2} \, dy\\ &= -\frac{\pi}{8} + \frac{1}{2} \int_0^1 \left (1 - \frac{1}{1 + y^2} \right ) \, dy\\ &= -\frac{\pi}{8} + \frac{1}{2} \left (1 - \frac{\pi}{4} \right )\\ &= \frac{1}{2} - \frac{\pi}{4}. \end{align*}

Explanation

(1) Interchanging the sum with the double integration.

(2) Summing the series which is geometric.

(3) Integration by parts.

• Thanks! How do you get rid of the $(-1)^{n}$ at (1)? – bencemeszaros Jan 20 '18 at 11:42
• As follows: $(-1)^n x^{2n} y^{2n} = (-1)^n (x^2)^n (y^2)^n = (-x^2y^2)^n$. – omegadot Jan 20 '18 at 22:40
