In this article, we obtain the general solution and prove the Hyers-Ulam stability of the following quadratic-multiplicative functional equation: $$\varphi (st-uv)+\varphi (sv+tu)=[\varphi (s)+\varphi (u)][\varphi (t)+\varphi (v)]$$ by using the direct method and the fixed point method.

The main aim of this paper is to establish the Hyers–Ulam stability and hyperstability of a Jensen-type quadratic mapping in 2-Banach spaces. That is, we prove the various types of Hyers–Ulam stability and hyperstability of the Jensen-type quadratic functional equation of the form $$ g \biggl( \frac{x+y}{2} + z \biggr) + g \biggl( \frac{x+y}{2} - z \biggr) + g \biggl( \frac{x-y}{2} + z \biggr) + g \biggl( \frac{x-y}{2} - z \biggr) = g(x) + g(y) + 4 g(z), $$g ( x + y 2 + z ) + g ( x + y 2 − z ) + g ( x − y 2 + z ) + g ( x − y 2 − z ) = g ( x ) + g ( y ) + 4 g ( z ) , in 2-Banach spaces by using the Hyers direct method.

In this paper we introduce and study near g-Riesz basis, Besselian g-frames and unconditional g-frames. We show that a near g-Riesz basis is a Besselian g-frame and we conclude that under some conditions the kernel of associated synthesis operator for a near g-Riesz basis is finite dimensional. Finally, we show that a g-frame is a g-Riesz basis for a Hilbert space H if and only if there is an equivalent inner product on H, with respect to which it becomes an g-orthonormal basis for H.

In this paper, we acquire the general solution of the generalized quadratic functional equation $ \begin{aligned} \sum\limits_{1 \leq a \lt b \lt c \leq m}\varphi\left(r_{a}+r_{b}+r_{c}\right)& = (m-2)\sum\limits_{1\leq a \lt b\leq m}\varphi\left(r_{a}+r_{b}\right) \\ &\quad-\left(\frac{m^{2}-3m+2}{2}\right)\nonumber \sum\limits_{a = 1}^{m}\frac{\varphi\left(r_{a}\right)+\varphi\left(-r_{a}\right)}{2} \end{aligned} $ where $m\geqslant 3$ is an integer. We also investigate Hyers-Ulam stability results by means of using alternative fixed point theorem for this generalized quadratic functional equation.