Stability of a generalized quadratic functional equation in various spaces: a fixed point alternative approach

Using the fixed point method, we prove the Hyers-Ulam stability of the following quadratic functional equationcf(∑ni=1xi)+∑nj=2f(∑ni=0xi−(n+c−1)xj)=(n+c−1)(f(x1)+c∑ni=2f(xi)+∑ni<j,j=3(∑n−1i=2f(xi−xj)))in various normed spaces.2010 Mathematics Subject Classification: 39B52; 46S40; 34K36; 47S40; 26E50; 47H10; 39B82