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.

Ivić, A. "On a Problem of Erdős Involving the Largest Prime Factor of ."
5 Nov 2003. http://arxiv.org/abs/math.NT/0311056.Taylor,
B. N. Guide for the Use of the International System of Units (SI). United
States Department of Commerce. NIST Special Publication 811, p. 33, 1995. http://physics.nist.gov/cuu/pdf/sp811.pdf.