There are several conflicting meanings associated with the notation . In German and Russian literature, the notation is used to mean the common logarithm . This is also the usage recommended by the United States Department of Commerce (Taylor 1995, p. 33).
However, is sometimes identified with the binary logarithm in some number theoretic literature (and here, mean the base-2 logarithm, not the nested natural logarithm as defined by Ivić 2003).
Great care is therefore needed to determine the intended definition for when it is encountered in the wild.