1. On the stability of cosine functional equation;Badora;Rocznik Nauk.-Dydakt. Prace Mat.,1998
2. On some trigonometric functional inequalities;Badora,2002
3. The stability of the cosine equation;Baker;Proc. Amer. Math. Soc.,1980
4. The stability of the equation f(x+y)=f(x)f(y);Baker;Proc. Amer. Math. Soc.,1979
5. On the cosine–sine functional equation on groups;Friis;Aequationes Math.,2002