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Given a map $f:\mathbb{R}^m\rightarrow \mathbb{R}^n$, is the condition that for all $x,y$ in $\mathbb{R}^m$, $f(x+y)=f(x)+f(y)$ enough for $f$ to be $\mathbb{R}$-linear? I cannot prove it or think of a counter example.

Jiu
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1 Answers1

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It should also satisfy:

$$f(ax)=af(x).$$

Ernie060
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