Summary.
In this paper, the invariance equation
$$(\varphi_{1} + \varphi_{2})^{-1} (\varphi_{1}(x) + \varphi_{2}(y)) + (\psi_{1} + \psi_{2})^{-1}(\psi_{1}(x) + \psi_{2}(y)) = x + y$$
is solved under four times continuous differentiability of the unknown functions φ1, φ2, ψ1, ψ2.
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This research was supported by the Hungarian Research Fund (OTKA) Grant Nos. K-62316, NK-68040.
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Baják, S., Páles, Z. Invariance equation for generalized quasi-arithmetic means. Aequ. math. 77, 133–145 (2009). https://doi.org/10.1007/s00010-008-2939-5
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DOI: https://doi.org/10.1007/s00010-008-2939-5