# famous Euler sum [duplicate]

Does anyone know how Euler in the 18th century proved that $$\sum_{n=1}^{\infty} \frac{H_n}{n^2}=2 \zeta(3)$$ with $$H_n$$ being the $$n$$'th harmonic number?

• Are you asking for Euler's proof specifically, or just any proof? – vrugtehagel Jun 20 '19 at 9:13
• I was actually asking for the classic approach. From the answers I reckon he did the more general formula with q instead of the square. – MikeGp Jun 20 '19 at 9:31
• The linked question explicitly asked for Euler's approach. – YuiTo Cheng Jun 20 '19 at 9:56