Given a continuous strictly monotonic real-valued function α, defined on an interval I, and a function ω:I→(0,+∞) we denote by Bωα the Bajraktarević mean generated by α and weighted by ω:
Bωα(x,y) = α-1(\frac{ω(x)}{ω(x)+ω(y)}α(x) + \frac{ω(y)}{ω(x)+ω(y)}α(y)), x,y∈I.
We find a necessary integral formula for all possible three times differentiable solutions (ϕ,ψ) of the functional equation
r(x)Bsϕ(x,y) + r(y)Btψ{t}(x,y) = r(x)x + r(y)y,
where r, s,t:I→(0,+∞) are three times differentiable functions and the first derivatives of ϕ,ψ and r do not vanish. However, we show that not every pair (ϕ,ψ) given by the found formula actually satisfies the above equation.
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