Let $f:\mathbb{R}\to\mathbb{R}$ a function differentiable $p$ times in $\mathbb{R}$, such that $f$ and $f^{(p)}$ bounded. Consequently, I would show that all intermediate derivatives $f^{(1)},...,f^{(p-1)}$ are also bounded on $\mathbb{R}$.
By myself, I succedeed in the case $p=2$ in this way:
$$f(x_0+h)=f(x_0)+f^{(1)}(x_0)h+f^{(2)}(\xi _1)\frac{h^2}{2}$$ $$f(x_0-h)=f(x_0)-f^{(1)}(x_0)h+f^{(2)}(\xi _2)\frac{h^2}{2}$$
that are true $\forall x_0\in\mathbb{R}$ and $\forall h>0$ with $\xi_1\in (x_0, x_0+h)$ and $\xi_2\in (x_0-h, x_0)$.
So subtracting the first equation and the second:
$$f(x_0+h)-f(x_0-h)=2hf^{(1)}(x_0)+\frac{h^2}{2}\left( f^{(2)}(\xi _1)-f^{(2)}(\xi _2) \right)$$
and so:
$$f^{(1)}(x_0)=\frac{f(x_0+h)-f(x_0-h)}{2h}-\frac{h}{4}\left( f^{(2)}(\xi _1)-f^{(2)}(\xi _2) \right)$$
taking the absolute value of both members and using the triangular inequality at right member:
$$|f^{(1)}(x_0)|\leq \frac{|f(x_0+h)|+|f(x_0-h)|}{2h}+\frac{h}{4}\left( |f^{(2)}(\xi _1)|+|f^{(2)}(\xi _2)| \right)$$
from which:
$$|f^{(1)}(x_0)|\leq \frac{M_{0}}{h}+\frac{h}{2}M_{2}$$
where $M_{0}:=\sup_{x\in\mathbb{R}}|f(x)|$ and $M_{2}:=\sup_{x\in\mathbb{R}}|f^{(2)}(x)|$.
Consequently:
$$\sup_{x\in\mathbb{R}}|f^{(1)}(x)|=:M_1\leq \frac{M_{0}}{h}+\frac{h}{2}M_{2}$$.
In a similar way I also proved the claim in the case $p=3$, but I can't start the induction chain.
Can you help me? Thanks.
