en.wikipedia.org/wiki/Entropic_uncertainty
3 corrections found
‖ F ‖ q , p = p 1 / p / q 1 / q .
This formula is missing a square root. For the 1D Fourier transform, Beckner’s sharp constant is \((p^{1/p}/q^{1/q})^{1/2}\), not \(p^{1/p}/q^{1/q}\).
Full reasoning
For the Fourier transform normalized as (\hat f(\xi)=\int_{\mathbb R} e^{-2\pi i x\xi}f(x),dx), the sharp Hausdorff–Young/Babenko–Beckner operator norm in one dimension is
[
|\mathcal F|_{q,p}=\left(\frac{p^{1/p}}{q^{1/q}}\right)^{1/2}, \qquad 1<p\le 2,\ \frac1p+\frac1q=1.
]
So the flat text here omits the square root and states a larger constant than the true one. A standard reference summary of Beckner’s sharp inequality gives the constant in dimension (n) as ((p^{1/p}(p')^{-1/p'})^{n/2}); in dimension 1 with (p'=q), that is exactly ((p^{1/p}/q^{1/q})^{1/2}).
2 sources
- Hausdorff-Young inequality - HandWiki
Beckner ... showed that if p is a number in the interval [1,2], then ... for any f in L^p(R^n). ... the operator norm ... is exactly equal to ( p^{1/p} (p′)^{-1/p′} )^{n/2}.
- Entropic uncertainty
Beckner proved that the value of this norm (in one dimension) for all q ≥ 2 is ... \|\mathcal F\|_{q,p}=\sqrt{p^{1/p}/q^{1/q}}.
H ( ϕ ) ≤ log 2 π e V ( ϕ )
This inequality is missing a factor of 1/2 (equivalently, a square root). The standard bound is \(H(\phi) \le \tfrac12\log(2\pi e\,V(\phi)) = \log\sqrt{2\pi e\,V(\phi)}\).
Full reasoning
The one-dimensional maximum-entropy bound for a density with variance (V(\phi)) is
[
H(\phi) \le \frac12\log\bigl(2\pi e,V(\phi)\bigr)
]
with equality for a Gaussian. The flat text here drops the factor (1/2) (or, equivalently, the square root inside the logarithm), changing the inequality.
Why this matters: for a Gaussian with variance (\sigma^2), the entropy is exactly (\tfrac12\log(2\pi e\sigma^2)). So the correct Gaussian-equality case is incompatible with the printed formula unless that missing factor is restored.
2 sources
- Properties of Differential Entropy and Related Measures
The differential entropy of a Gaussian with variance σ^2 is (1/2)log( 2πe σ^2 ). The Gaussian distribution has the highest entropy among all distributions with the same variance.
- Differential entropy - HandWiki
If a random vector X in R^n has mean zero and covariance matrix K, h(X) ≤ 1/2 log[(2πe)^n det K] with equality if and only if X is jointly gaussian.
1 / 2 ≤ exp ( H ( | f | 2 ) + H ( | g | 2 ) ) / ( 2 e π ) ≤ V ( | f | 2 ) V ( | g | 2 )
The right-hand side should be the square root of the variance product, not the product itself. The standard inequality is \(\frac12 \le e^{H(|f|^2)+H(|g|^2)}/(2\pi e) \le \sqrt{V(|f|^2)V(|g|^2)}\).
Full reasoning
Combining the entropic uncertainty bound with the Gaussian maximum-entropy bound gives
[
\frac12 \le \frac{\exp(H(|f|^2)+H(|g|^2))}{2\pi e} \le \sqrt{V(|f|^2)V(|g|^2)}.
]
So the flat text is missing the square root on the right.
This is not just a cosmetic typo: the printed version would imply the stronger lower bound (V(|f|^2)V(|g|^2)\ge 1/2), which is false. For minimum-uncertainty Gaussians, equality holds in the correct inequality and one has (\sqrt{V(|f|^2)V(|g|^2)}=1/2), i.e. (V(|f|^2)V(|g|^2)=1/4), not (\ge 1/2).
2 sources
- A Refinement of the Robertson-Schrödinger Uncertainty Principle and a Hirschman-Shannon Inequality for Wigner Distributions
we may combine (18) and (19) and obtain ... log(πħe)^n ≤ E(|f|^2)+E(|F_ħ f|^2) ≤ log[(2πe)^n sqrt(det(Cov(|f|^2)) · det(Cov(|F_ħ f|^2)))] ... equalities throughout if and only if f is a Gaussian.
- Entropic uncertainty
1/2 ≤ exp(H(|f|^2)+H(|g|^2))/(2eπ) ≤ sqrt(V(|f|^2)V(|g|^2)).