In this article, for a certain subset $\mathcal{ X}$ of the extended set of rational numbers, we introduce the notion of { \it best $\mathcal{ X}$-approximations} of a real number. The notion of best $\mathcal{ X}$-approximation is analogous to that of best rational approximation. We explore these approximations with the help of $\mathcal{ F}_{ p^l}$-continued fractions, where $p$ is a prime and $l\in\mathbb{ N}$, we show that the convergents of the $\mathcal{ F}_{ p^l}$-continued fraction expansion of a real number $x$ satisfying certain maximal conditions are exactly the best $\mathcal{ F}_{ p^l}$-approximations of $x$.
Categories: math.NT