On functional equation $\varphi\varphi(x)=x$, $x\in(-\infty,\infty)$.
(English).Acta Universitatis Palackianae Olomucensis. Facultas Rerum Naturalium. Mathematica,
vol. 31
(1992),
issue 1,
pp. 83-100
[1] Kuczma M.: Functional equation in a single variable. Polska akademia nauk, Monografie matematyczne, Tom 4, PWN - Polish scientific publishers, Warszawa 1968. MR 0228862