On Ulam Stability and Hyperstability of Functional Equations in Banach Spaces and 2-Banach Spaces
摘要
We investigate the Ulam stability and hyperstability of the functional equations \(\displaystyle \begin {array}{l} f\left (\frac {x-y}{2}+z\right )+ f\left (\frac {y-z}{2}+x\right )+ f\left (\frac {z-x}{2}+y\right )=f(x+y+z),\\ f\left (\frac {x-y}{2}+z\right )+ f\left (\frac {y-z}{2}+x\right )+ f\left (\frac {z-x}{2}+y\right )=f(x)+f(y)+f(z), \end {array} \) in Banach spaces and 2-Banach spaces.