MHB Converging to the Basel Problem: Solving for Poles on the Real Axis

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The discussion centers on various methods to solve the Basel problem, which states that the sum of the reciprocals of the squares of natural numbers equals π²/6. Euler's use of Bernoulli numbers is highlighted, but alternative approaches are also explored, particularly involving residues and analytic functions. The conversation emphasizes adjusting theorems to account for poles on the real axis, specifically addressing the pole of order 2 at the origin. The modified theorem leads to a confirmation of the Basel problem's result. The author offers to share the proof of the theorem's modification if there is interest.
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$$\sum_{k\geq 1} \frac{1}{k^2} = \frac{\pi^2}{6}$$

Let us see how many different methods can we get (Cool)
 
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Euler used the Bernoulli numbers to solve the problem

But there are other ways

Fourier series , Complex analysis
 
$$\sum_{k = -\infty}^{\infty} f(k) = - \sum_{i\geq 0} \text{Res}\, \left( \pi \cot(\pi z) f(z);z_i \right) $$

This requires the function to be analytic on the real axis but $$\frac{1}{k^2}$$ has a pole of order $$2$$ at the origin .

So we can adjust the theorem to solve for poles on the real axis

$$\sum_{k\leq -1} \frac{1}{k^2} +\sum_{k \geq 1} \frac{1}{k^2}= - \text{Res}\, \left(\frac{\pi \cot(\pi z)}{z^2};0 \right)$$

$$\sum_{k\geq 1} \frac{1}{k^2} +\sum_{k\geq 1} \frac{1}{k^2}= \frac{\pi^2}{3}$$

$$\sum_{k\geq 1} \frac{1}{k^2} = \frac{\pi^2}{6}$$

I deleted the proof of the modification of the theorem above . If someone is interested I will try to post it.
 
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