I think that the first paragraph of the chapter 2.2.3. page 301 clarifies the issue, see http://books.google.pl/books?id=Yk4i8HvFm_IC&pg=PA302&lpg=PA302&dq=ostrowski+theorem&source=bl&ots=kOPiDK6VGH&sig=vrc7_OB-GatbculwXwHTocg76Jo&hl=pl&sa=X&ei=1lieUcmVKIKPtAbOqYGgAg&ved=0CCcQ6AEwADgU#v=onepage&q=ostrowski%20theorem&f=false The reference is: Ostrowski, Alexander (1918). "Über einige Lösungen der Funktionalgleichung φ(x)·φ(y)=φ(xy)". Acta Mathematica (2nd ed.) 41 (1): 271–284 although the paper is dated April 1916. That's why wikipedia says Ostrowski's theorem, due to Alexander Ostrowski (1916).
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