Approximation by superposition of sigmoidal and radial basis functions

Let σ: R → R be such that for some polynomial P, σ P is bounded. We consider the linear span of the functions {σ(λ · (x - t)): λ, t ε{lunate} Rs}. We prove that unless σ is itself a polynomial, it is possible to uniformly approximate any continuous function on Rs arbitrarily well on every compact subset of Rs by functions in this span. Under more specific conditions on σ, we give algorithms to achieve this approximation and obtain Jackson-type theorems to estimate the degree of approximation. © 1992.