Equation
[1] f(x+y) + f (f(x)+f(y)) = f (f(x+f(y)) + f(f(x)+y))
has been proposed by C. Alsina in the class of continuous and decreasing involutions of (0,+∞). General solution of [1] is not known yet. Nevertheless we give solutions of the following equations which may be derived from [1]:
[2] f(x+1) + f (f(x)+1) = 1,
[3] f(2x) + f(2f(x)) = f(2f(x + f(x))).
Equation [3] leads to a Cauchy functional equation:
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