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Measurable functionals on function spaces

J. P. Reus Christensen, J. K. Pachl (1981)

Annales de l'institut Fourier

We prove that all measurable functionals on certain function spaces are measures; this improves the (known) results about weak sequential completeness of spaces of measures. As an application, we prove several results of this form: if the space of invariant functionals on a function space is separable then every invariant functional is a measure.

Metric spaces admitting only trivial weak contractions

Richárd Balka (2013)

Fundamenta Mathematicae

If (X,d) is a metric space then a map f: X → X is defined to be a weak contraction if d(f(x),f(y)) < d(x,y) for all x,y ∈ X, x ≠ y. We determine the simplest non-closed sets X ⊆ ℝⁿ in the sense of descriptive set-theoretic complexity such that every weak contraction f: X → X is constant. In order to do so, we prove that there exists a non-closed F σ set F ⊆ ℝ such that every weak contraction f: F → F is constant. Similarly, there exists a non-closed G δ set G ⊆ ℝ such that every weak contraction...

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