1. Moreno Samuel , A one-sentence and truly elementary proof of the Basel Problem, available at https://arxiv.org/pdf/1502.07667.pdf
2. Inside Interesting Integrals
3. Euler's Other Proof
4. Chapman Robin , Evaluating ζ (2), available at http://empslocal.ex.ac.uk/people/staff/rjchapma/rjc.html
5. Another Eulerian-Type Proof