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2009 Jozsef Wildt International Math Competition, W. 28

Let $\theta$ and $p(p<1)$ ) be nonnegative real numbers. Suppose that $f:X\to Y$ is mapping with $f(0)=0$ and $$\left |\left| 2f\left (\frac{x+y}{2}\right )-f(x)-f(y) \right |\right|_Y \leq \theta\left (\left |\left |x\right |\right |_X^p +\left |\left |y\right |\right |_X^p \right )$$ for all $x$, $y\in \mathbb{Z}$ with $x\perp y$ where $X$ is an orthogonality space and $Y$ is a real Banach space. Prove that there exists a unique orthogonally Jensen additive mapping $T:X\to Y$, namely a mapping $T$ that satisfies the so-called orthogonally Jensen additive functional equation $$2f\left (\frac{x+y}{2}\right )=f(x)+f(y)$$for all $x$, $y\in \mathbb{X}$ with $x\perp y$, satisfying the property $$\left |\left|f(x)-T(x) \right |\right|_Y \leq \frac{2^p\theta}{2-2^p}\left |\left |x\right |\right |_X^p$$ for all $x\in X$