Decomposing Functions into Bounded and Lipschitz Parts

fa.functional-analysisreal-analysis

Let $f: \mathbb R^d \to \mathbb R^d$ be a measurable function such that
$$
\sup_{x,y \in \mathbb R^d} \frac{|f(x) – f(y)|}{\max \{1, |x-y| \}} < \infty.
$$

Are there functions $g,h: \mathbb R^d \to \mathbb R^d$ such that $f = g + h$ and
$$
\begin{align*}
\sup_{\substack{x,y \in \mathbb R^d \\x \neq y}} \frac{|g(x) – g(y)|}{|x-y|} + \sup_{x \in \mathbb R^d} |h(x)| &< \infty.
\end{align*}
$$

?

Thank you so much for your elaboration!

Best Answer

Sure. Triangulate the unit cube $[0,1]^d$ into $d!$ simplices $S_1$, $\ldots$, $S_{d!}$, as in any of the answers to this question.

Let $g$ satisfy $g(\vec{n}) = f(\vec{n})$ for all $\vec{n} \in \mathbb{Z}^d$, and make it linear on $S_i + \vec{n}$ for every $1 \leq i \leq d!$ and $\vec{n} \in \mathbb{Z}$. Then $g$ is Lipschitz and $f - g$ is bounded.