For each ordinal ξ and each 1 q < ∞, we define the notion of ξ-q-summable Szlenk index. When ξ = 0 and q = 1, this recovers the usual notion of summable Szlenk index. We define for an arbitrary weak * -compact set a transfinite, asymptotic analogue α ξ,p of the martingale type norm of an operator. We prove that this quantity is determined by norming sets and determines ξ-Szlenk power type and ξ-q-summability of Szlenk index. This fact allows us to prove that the behavior of operators under the α ξ,p seminorms passes in the strongest way to injective tensor products of Banach spaces. Furthermore, we combine this fact with a result of Schlumprecht to prove that a separable Banach space with good behavior with respect to the α ξ,p seminorm can be embedded into a Banach space with a shrinking basis and the same behavior under α ξ,p , and in particular it can be embedded into a Banach space with a shrinking basis and the same ξ-Szlenk power type. Finally, we completely elucidate the behavior of the α ξ,p seminorms under ℓ r direct sums. This allows us to give an alternative proof of a result of Brooker regarding Szlenk indices of ℓ p and c 0 direct sums of operators.2010 Mathematics Subject Classification. Primary: 46B03, 46B06; Secondary: 46B28, 47B10.