The frac, dfrac, and tfrac commands

The frac command, which is in the basic command set of /,  takes two arguments---numerator and denominator---and typesets them in normal fraction form. The amsmath package provides also dfrac and tfrac as convenient abbreviations for | |...| || and    | |...| ||.

\begin{equation}
\frac{1}{k}\log_2 c(f)\;\tfrac{1}{k}\log_2 c(f)\;
\sqrt{\frac{1}{k}\log_2 c(f)}\;\sqrt{\dfrac{1}{k}\log_2 c(f)}
\end{equation}