## Abstract

In our paper [Proc. Amer. Math. Soc. Ser. B 8 (2021), pp. 86–99] we showed that a Tychonoff space X is a Δ-space (in the sense of R. W. Knight [Trans. Amer. Math. Soc. 339 (1993), pp. 45–60], G. M. Reed [Fund. Math. 110 (1980), pp. 145–152]) if and only if the locally convex space

that T admits an extension to a linear continuous operator T from R

RY and deduce that

X and Y are metrizable spaces, we show that Y is a

*Cp(X)*is distinguished. Continuing this research, we investigate whether the class Δ of Δ-spaces is invariant under the basic topological operations. We prove that if X ∈ Δ and ϕ : X → Y is a continuous surjection such that ϕ(F) is an Fσ-set in Y for every closed set F ⊂ X, then also Y ∈ Δ. As a consequence, if X is a countable union of closed subspaces Xi such that each Xi ∈ Δ, then also X ∈ Δ. In particular, σ-product of any family of scattered Eberlein compact spaces is a Δ-space and the product of a Δ-space with a countable space is a Δ-space. Our results give answers to several open problems posed by us [Proc. Amer. Math. Soc. Ser. B 8 (2021), pp. 86–99]. Let*T*:*Cp(X)*−→*Cp(Y )*be a continuous linear surjection. We observethat T admits an extension to a linear continuous operator T from R

^{X}ontoRY and deduce that

*Y*is a Δ-space whenever X is. Similarly, assuming thatX and Y are metrizable spaces, we show that Y is a

*Q*-set whenever X is. Making use of obtained results, we provide a very short proof for the claim that every compact Δ-space has countable tightness. As a consequence, under Proper Forcing Axiom every compact Δ-space is sequential. In the article we pose a dozen open questionsOriginal language | English |
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Pages (from-to) | 267-280 |

Journal | Proceedings of the American Mathematical Society |

Volume | 8 |

DOIs | |

State | Published - 21 Sep 2021 |

## Keywords

- Mathematics - General Topology

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