Let fi(xi) be non-constant
rational functions over a field K (i = 1,2,…,n). A necessary and sufficient condition
is given for reducibility over K of the numerator of the sum ∑i=1nfi(xi) in its
reduced form, provided n ≥ 3. In particular the numerator is irreducible if
charK = 0, which generalizes a theorem of Ehrenfeucht and Pełczyński and answers
a question of M. Jarden.