Two exercises use $\mu$ to mean the mean parameter of a normal distribution. However, nowhere in the book is it defined as such. For clarity replace $\mu$ with "mean".
The question asks:
> Follow <http://nicercode.github.io/intro/writing-functions.html> to write your own functions to compute the variance and skew of a numeric vector.
However, the math on that page no longer renders.
Directly include the skew and variance equations in the question.
Jeffrey ArnoldandHadley Wickham