Log[z] gives the natural logarithm of z (logarithm to base ).
Log[b, z] gives the logarithm to base b.
Mathematical function (see Section A.3.10).
Log gives exact rational number results when possible.
Log[z] has a branch cut discontinuity in the complex z plane running from to .
See The Mathematica Book: Section 1.1.3 and Section 3.2.6.
See also: Exp, Power, Arg, MantissaExponent, ProductLog, HarmonicNumber.