What is the log base 2 of a negative number?
With real numbers you cannot have the logarithm of a negative number.
With complex numbers
$$\log_2{(-m)}= \log_2{(m)}+\frac{(2n+1)\pi}{\ln2}*i$$
where m is positive and n is any (positive or negative) integer.
I want to stress Alan's first answer.
