Showing that $T+S$ is firmly nonexpansive

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Show that $T+S$ is firmly nonexpansive considering that $T$ and $S$ are firmly nonexpansive mappings from $\mathbb R^n$ to $\mathbb R^n$.

Definition: We say that $F$ is firmly nonexpansive if: $$\|F(x)-F(y)\|^2+\|(I-F)(x)-(I-F)(y)\|^2\le \|x-y\|^2$$

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It is false. Let $T = S = I$. Then $T+S = 2I$ which once you plug in you see that is not firmly nonexpansive. Do you perhaps mean to consider $\frac12(T+S)$? In which case the desired conclusion follows from triangle inequality applied many times.