Retract or Not: A Tale of Two Fans

Abstract

Let $X$ be a Lelek fan or a Cantor fan and let $Y$ be a Lelek fan or a Cantor fan. In this paper, we study embeddings $f:X\to Y$ that admit retractions from $Y$ onto $f(X)$. In 1989, W. J. Charatonik and J. J. Charatonik proved that if $X$ is a Lelek fan and $Y$ is a Cantor fan, then no embedding $f$ of $X$ into $Y$ admits a retraction from $Y$ onto $f(X)$. They also showed that if both $X$ and $Y$ are Cantor fans, then every embedding $f$ of $X$ into $Y$ admits such a retraction. In this paper, we address the two remaining cases. First, we consider the situation where $X$ is a Cantor fan and $Y$ is a Lelek fan. We prove that in this case, every embedding $f$ of $X$ into $Y$ admits a retraction from $Y$ onto $f(X)$. Second, we examine the case where both $X$ and $Y$ are Lelek fans. Here, we show that there exist embeddings $f$ that do admit a retraction from $Y$ onto $f(X)$, as well as embeddings that do not. For this latter case, we also identify additional properties of embeddings that ensure the existence of a retraction from $Y$ onto $f(X)$.