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(Very) Minor Math Jaxing: used $\|\cdot\|$ instead of $||\cdot||$
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Daniele Tampieri
Daniele Tampieri
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I'm studying harmonic analysis by myself, and I read some online notes that introduce the Bernstein inequality. One of them mention a reversed form of the Bernstein inequality, which is stated below:

Let $\mathbb{T} = \mathbb{R} / \mathbb{Z} = [0,1]$ be the one-dimensional torus. Assume that a function $f \in L^{1}(\mathbb{T})$ satisfies $\hat{f}(j) = 0$ for all $|j| < n$ (vanishing Fourier coefficients). Then for all $1 \leq p \leq \infty$, there exists some constant $C$ independent of $n,p$ and $f$, such that $$||f'||_{p} \geq Cn||f||_{p}$$$$\|f'\|_{p} \geq Cn\|f\|_{p}$$

It seems that an easier problem can be obtained by replacing $f'$ with $f''$ in the above inequality. The easier problem is addressed in the MO post below:

Does there exist some $C$ independent of $n$ and $f$ such that $ \|f''\|_p \geq Cn^2 \| f \|_p$, where $1 \leq p\leq \infty$?

However, it seems that the trick of convex Fourier coefficients used in the post above no longer applies to the harder problem (lower bounding the norm of the first derivative). Any suggestions/ideas?

I'm studying harmonic analysis by myself, and I read some online notes that introduce the Bernstein inequality. One of them mention a reversed form of the Bernstein inequality, which is stated below:

Let $\mathbb{T} = \mathbb{R} / \mathbb{Z} = [0,1]$ be the one-dimensional torus. Assume that a function $f \in L^{1}(\mathbb{T})$ satisfies $\hat{f}(j) = 0$ for all $|j| < n$ (vanishing Fourier coefficients). Then for all $1 \leq p \leq \infty$, there exists some constant $C$ independent of $n,p$ and $f$, such that $$||f'||_{p} \geq Cn||f||_{p}$$

It seems that an easier problem can be obtained by replacing $f'$ with $f''$ in the above inequality. The easier problem is addressed in the MO post below:

Does there exist some $C$ independent of $n$ and $f$ such that $ \|f''\|_p \geq Cn^2 \| f \|_p$, where $1 \leq p\leq \infty$?

However, it seems that the trick of convex Fourier coefficients used in the post above no longer applies to the harder problem (lower bounding the norm of the first derivative). Any suggestions/ideas?

I'm studying harmonic analysis by myself, and I read some online notes that introduce the Bernstein inequality. One of them mention a reversed form of the Bernstein inequality, which is stated below:

Let $\mathbb{T} = \mathbb{R} / \mathbb{Z} = [0,1]$ be the one-dimensional torus. Assume that a function $f \in L^{1}(\mathbb{T})$ satisfies $\hat{f}(j) = 0$ for all $|j| < n$ (vanishing Fourier coefficients). Then for all $1 \leq p \leq \infty$, there exists some constant $C$ independent of $n,p$ and $f$, such that $$\|f'\|_{p} \geq Cn\|f\|_{p}$$

It seems that an easier problem can be obtained by replacing $f'$ with $f''$ in the above inequality. The easier problem is addressed in the MO post below:

Does there exist some $C$ independent of $n$ and $f$ such that $ \|f''\|_p \geq Cn^2 \| f \|_p$, where $1 \leq p\leq \infty$?

However, it seems that the trick of convex Fourier coefficients used in the post above no longer applies to the harder problem (lower bounding the norm of the first derivative). Any suggestions/ideas?

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geooranalysis
geooranalysis
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"Reversed" Bernstein Inequality

I'm studying harmonic analysis by myself, and I read some online notes that introduce the Bernstein inequality. One of them mention a reversed form of the Bernstein inequality, which is stated below:

Let $\mathbb{T} = \mathbb{R} / \mathbb{Z} = [0,1]$ be the one-dimensional torus. Assume that a function $f \in L^{1}(\mathbb{T})$ satisfies $\hat{f}(j) = 0$ for all $|j| < n$ (vanishing Fourier coefficients). Then for all $1 \leq p \leq \infty$, there exists some constant $C$ independent of $n,p$ and $f$, such that $$||f'||_{p} \geq Cn||f||_{p}$$

It seems that an easier problem can be obtained by replacing $f'$ with $f''$ in the above inequality. The easier problem is addressed in the MO post below:

Does there exist some $C$ independent of $n$ and $f$ such that $ \|f''\|_p \geq Cn^2 \| f \|_p$, where $1 \leq p\leq \infty$?

However, it seems that the trick of convex Fourier coefficients used in the post above no longer applies to the harder problem (lower bounding the norm of the first derivative). Any suggestions/ideas?