On the functional equation f ( x + y ) f ( x - y ) = f 2 ( x ) - f 2 ( y ) S. Kurepa — 1961 Annales Polonici Mathematici
Some Relations Between Additive Functions, II. (Short Communication). Pl. Kannappan; S. Kurepa — 1970 Aequationes mathematicae
Some Relations Between Additive Functions - II PL. KANNAPPAN; S. KUREPA — 1971 Aequationes mathematicae
Some Relations Between Additive Function - I. (Short Communication). Pl. Kannappan; S. Kurepa — 1969 Aequationes mathematicae
Some Relations Between Additive Functions - I. Pl. Kannappan; S. Kurepa — 1970 Aequationes mathematicae