On countable choice and sequential spaces

Under the axiom of choice, every first countable space is a FréchetUrysohn space. Although, in its absence even R may fail to be a sequential space. Our goal in this paper is to discuss under which set-theoretic conditions some topological classes, such as the first countable spaces, the metric spaces or the subspaces of R, are classes of Fréchet-Urysohn or sequential spaces. In this context, it is seen that there are metric spaces which are not sequential spaces. This fact arises the question of knowing if the completion of a metric space exists and it is unique. The answer depends on the definition of completion. Among other results it is shown that: every first countable space is a sequential space if and only if the axiom of countable choice holds; the sequential closure is idempotent in R if and only if the axiom of countable choice holds for families of subsets of R; every metric space has a unique σ̂-completion.