[1BX] Letβs go back to the exercise [0FP]: computing the Cauchy product of the series \(β_{n=1}^β \frac{(-1)^{n-1}}{\sqrt{n}}\) with itself, produces the series \(β_ n (-1)^ n c_ n\) with \(c_ n = β_{k=1}^{n-1} \frac 1{\sqrt{k(n-k)}}\); show that \(c_ nβ π\).