Characterization of quasi-arithmetic means without regularity condition

In this paper we show that bisymmetry, which is an algebraic property, has a regularity improving feature. More precisely, we prove that every bisymmetric, partially strictly monotonic, reflexive and symmetric function $F:I^2\to I$ is continuous. As a consequence, we obtain a finer characterization of quasi-arithmetic means than the classical results of Acz\'el, Kolmogoroff, Nagumo and de Finetti.