Fonction quantile

Résumé

En probabilités, la fonction quantile est une fonction qui définit les quantiles. Définition formelle Soit X une variable aléatoire et F sa fonction de répartition, la fonction quantile est définie par : Q(q) = F^{\leftarrow}(q) = \inf \left{x : F(x) \geqslant q \right} pour toute valeur de q \in [0,1], la notation F^{\leftarrow} désignant l’inverse généralisé à gauche de F. Si F est une fonction strictement croissante et continue, alors Q(q) est l'unique valeur de x telle que F(x) = q. F^{\leftarrow} correspond alors à la fonction réciproque de F, notée F^{-1}. En revanche, pour les lois discrètes, les fonctions de répartition sont toutes en escalier, d'où l'intérêt de la définition précédente. On dit que :

Q(0,!5) est la médiane ;
Q(0,!25) le premier quartile ;
Q(0,!7

Source officielle

https://fr.wikipedia.org/wiki/Fonction_quantile

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Scale anomalies, states, and rates in conformal field theory

Marc Gillioz

This paper presents two methods to compute scale anomaly coefficients in conformal field theories (CFTs), such as the c anomaly in four dimensions, in terms of the CFT data. We first use Euclidean position space to show that the anomaly coefficient of a four-point function can be computed in the form of an operator product expansion ( OPE), namely a weighted sum of OPE coefficients squared. We compute the weights for scale anomalies associated with scalar operators and show that they are not positive. We then derive a different sum rule of the same form in Minkowski momentum space where the weights are positive. The positivity arises because the scale anomaly is the coefficient of a logarithm in the momentum space four-point function. This logarithm also determines the dispersive part, which is a positive sum over states by the optical theorem. The momentum space sum rule may be invalidated by UV and/or IR divergences, and we discuss the conditions under which these singularities are absent. We present a detailed discussion of the formalism required to compute the weights directly in Minkowski momentum space. A number of explicit checks are performed, including a complete example in an 8-dimensional free field theory.

2017

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Fermion conformal bootstrap in 4d

Denis Karateev, Alessandro Vichi

We apply numerical conformal bootstrap techniques to the four-point function of a Weyl spinor in 4d non-supersymmetric CFTs. We find universal bounds on operator dimensions and OPE coefficients, including bounds on operators in mixed symmetry representations of the Lorentz group, which were inaccessible in previous bootstrap studies. We find discontinuities in some of the bounds on operator dimensions, and we show that they arise due to a generic yet previously unobserved fake primary effect, which is related to the existence of poles in conformal blocks. We show that this effect is also responsible for similar discontinuities found in four-fermion bootstrap in 3d, as well as in the mixed-correlator analysis of the 3d Ising CFT. As an important byproduct of our work, we develop a practical technology for numerical approximation of general 4d conformal blocks.

Springer2019

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A scattering amplitude in Conformal Field Theory

Marc Gillioz, Marco Meineri

We define form factors and scattering amplitudes in Conformal Field Theory as the coefficient of the singularity of the Fourier transform of time-ordered correlation functions, as p(2) -> 0. In particular, we study a form factor F(s, t, u) obtained from a four-point function of identical scalar primary operators. We show that F is crossing symmetric, analytic and it has a partial wave expansion. We illustrate our findings in the 3d Ising model, perturbative fixed points and holographic CFTs.

SPRINGER2020

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