1. V I Arnold, Problems for Kids from 5 to 15, MCNMO, Moscow, 2004, (in Russian).
2. A M Yaglom and I M Yaglom, Elementary Presentation of Nonelementary Problems, problem 143, Gostekhizdat, Moscow, 1954, (in Russian).
3. D Kalman, Six Ways to Sum a Series, College Math. J., Vol.24, pp.402–421, 1993.
4. K P Kokhas, Sum of inverse squares, Mathematicheskoe Prosveshenie, Vol.8, pp.142–163, 2004, (in Russian). See also informal notes by R Chapman, Evaluating ζ(2), http://www.secamlocal.ex.ac.uk/~rjc/etc/ zeta2.pdf
5. J Hofbauer, A simple proof of $$1 + \frac{1}{{{2^2}}} + \frac{1}{{{3^2}}} + \ldots = \frac{{{\pi ^2}}}{6}$$ and related identities, Am. Math. Mon, Vol.109, pp.196–200, 2002.