We consider the Carleson embeddings of the classical Hardy spaces (on the disk) into a L p (µ) space, where µ is a Carleson measure on the unit disk. This includes the case of composition operators. We characterize such operators which are r-summing on H p , where p > 1 and r ≥ 1. This completely extends the former results on the subject and solves a problem open since the early seventies.Mathematics Subject Classification. Primary: 30H10; 47B10 -Secondary: 30H20; 47B33
Key-words. composition operator -absolutely summing operators -Hardy spaces -Carleson measuresHere λ stands for the normalized Haar measure on the torus (it is the normalized arc length), and f r (z) = f (rz) with r ∈ (0, 1) and z ∈ D. Now, let us turn to our main subject. Given a positive Borel measure µ on the closed unit disk D, we consider the formal identity J µ from the Hardy space H p into L p (µ) (we keep the notation J µ instead of J p,µ in the sequel for sake of lightness):Thanks to a famous result of Carleson (see [C]), this is well defined and bounded if and only if µ is a Carleson measure, i.e. sup ξ∈T µ W(ξ, h) = O(h), when h → 0, * Partially supported by the project MTM2015-63699-P (Spanish MINECO and FEDER funds)Actually the best admissible C is (π r (T )) r .Very few results are known on absolutely summing composition operators: there is a characterization of r-summing composition operators on H p due to Shapiro and Taylor in [ST] only when r = p ≥ 2. The same result (with an obviously adapted proof) is actually valid for general Carleson embeddings:( 1.2) Moreover, for every p ≥ 1, the condition (1.2) is sufficient to ensure that J µ is a p -summing operator on H p . When 1 ≤ p ≤ 2, J µ is actually even absolutely summing since H p has cotype 2.A natural question then arises: is (1.2) the good condition (i.e. a necessary condition) when 1 ≤ p ≤ 2? This is false in general: Domenig proved in [Do] that, given p ∈ [1, 2) there exists an absolutely summing composition operator on H p which is not order bounded. He was able to give some sufficient condition for the construction of his example, but without any characterization. Let us mention that, in this case, it is equivalent to be an order bounded Carleson embedding and to verify condition (1.2). Indeed, the following is known from the specialists (see for instance [LLQR1] in the case of composition operators). Proposition 1.3 Let µ be a Carleson measure on the open unit disk D and p ≥ 1. J µ : H p → L p (µ) is order bounded if and only if (1.2) is satisfied.Proof. By definition, J µ is order bounded if and only if there exists some h ∈ L p (µ) such that for every f in the unit ball of H p , we have |f | ≤ h a.e. on D. Since H p is separable, it suffices to test this control on a dense countable subset of the unit ball de H p . Hence J µ is order bounded if and only if