, Volume 71, Issue 1-2, pp 70-85

# Invariance of the arithmetic mean with respect to special mean-type mappings

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## Summary.

For a continuous and strictly monotonic function f defined on an interval I of positive reals, the function $$M_{f} {\left( {x,y} \right)} = f^{{ - 1}} {\left( {\frac{{xf(x) + yf(y)}} {{x + y}}} \right)},x,y\, \in {I}$$ is a mean. Assuming that at least one of f and g is four times continuously differentiable, we prove that if the arithmetic mean A is (M f ,M g )-invariant, i.e. if A ○ (M f ,M g ) = A on I, then $$f(x) = C_{1} x^{{ - 1}} + B_{1} \,{\text{and}}\,{\text{g(x) = C}}_{{\text{2}}} x^{{ - 1}} + B_{2} ,$$ for some $$C_{{\text{1}}} ,C_{2} ,B_{1} ,B_{{\text{2}}} , \in \mathbb{R},C_{1}C_{2} \ne 0$$ . It remains an open question, if the smoothness conditions on the functions f and g can be relaxed.

Manuscript received: February 4, 2004 and, in final form, December 23, 2004.