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math.FA2005
Positive definite kernels and lattice paths
T. Constantinescu, Nermine El-Sissi
We discuss the structure of positive definite kernels in terms of operator models. In particular, we introduce two models, one of Hessenberg type and another one that we call near…
math.FA2004
Tensor algebras and displacement structure. IV. Invariant kernels
T. Banks, T. Constantinescu, Nermine El-Sissi
In this paper we investigate the class of invariant positive definite kernels on the free semigroup on N generators. We provide a combinatorial description of the positivity of the…