arXiv subjects
Publications and source records attributed to Alexis Drouot.
We provide sharp forms of $k$-plane transform inequalities on the $d$-dimensional sphere $\mathbb{S}^d$ and the $d$-dimensional hyperbolic space $\mathbb{H}^d$. In particular, we prove that extremizers do not exist for $\mathbb{H}^d$. This work is a natural extension of some results for the $k$-plane transform on $\mathbb{R}^d$.
The kinetic Brownian motion on the cosphere bundle of a Riemannian manifold $\mathbb{M}$ is a stochastic process that models the geodesic equation perturbed by a random white force of size $\varepsilon$. When $\mathbb{M}$ is compact with negative curvature we show that the $L^2$-spectrum of the infinitesimal generator of this process converges to the Pollicott--Ruelle resonances of $\mathbb{M}$ as $\varepsilon \rightarrow 0$.
We study resonances of compactly supported potentials $ V_\varepsilon = W ( x, x/\varepsilon ) $ where $ W : \mathbb{R}^d \times \mathbb{R}^d / ( 2π\mathbb{Z}) ^d \to \mathbb{C} $, $ d $ odd. That means that $ V_\varepsilon $ is a sum of a slowly varying potential, $ W_0 ( x) $, and one oscillating at frequency $1/\varepsilon$. For $ W_0 \equiv 0 $ we prove that there are no resonances above the line $\text{Im} λ= -A \ln(\varepsilon^{-1})$, except possibly a simple resonance of modulus $\sim \varepsilon^2$, when $ d=1$. We show that this result is optimal by constructing a one-dimensional example. In the case when $ W_0 \neq 0 $ we prove that resonances in fixed strips admit an expansion in powers of $\varepsilon$. The argument provides a method for computing the coefficients of the expansion. In particular we produce an effective potential converging uniformly to $W_0$ as $\varepsilon \rightarrow 0$ and whose resonances approach resonances of $V_\varepsilon$ modulo $O(\varepsilon^4)$.
We present a proof of the existence of the Hawking radiation for massive bosons in the Schwarzchild-de Sitter metric. It provides estimates for the rates of decay of the initial quantum state to the Hawking thermal state. The arguments in the proof include a construction of radiation fields by conformal scattering theory; a semiclassical interpretation of the blueshift effect; and the use of a WKB parametrix near the surface of a collapsing star. The proof does not rely on the spherical symmetry of the spacetime.
Let d > 1 and 0 < k < d. The k-plane transform satisies some Lp to Lq dilation-invariant inequality. In this case the best constant and the extremizers are explicitly known. We give a quantitative form of the inequality with respect to these extremizers, that works for k = d - 1 and for k < d-1 while restricted to radial functions.
A theorem proved by Quillen and by Catlin and D'Angelo states that a bi-homogeneous form on a multidimensional complex space which is positive away from zero can be written as a sum of squares of absolute values of polynomials once it is multiplied by the norm raised to a sufficiently high even power. In this note we provide a quantitative version of this theorem by giving an upper bound on the minimal power. This bound is roughly C_f (n+m)^3 log(n)^3, where n is the dimension and m the degree of the form, and C_f is a multiplicative constant depending only on f, inversely proportional to the minimum of f on the sphere.
The k-plane transform acting on test functions on R^d satisfies a dilation-invariant L^p to L^q inequality for some exponents p,q. We will explicit some extremizers and the value of the best constant for any value of k and d, solving the limit case of a 1997 conjecture from Baernstein and Loss. This extends their own result for k=2 and Christ's result for k=d-1.