A Wi-Fi password hidden in statistics expression You can see the image which is very popular in social media currently.
We want to know the password before some bad guys attack it :)
I think the expression is written using statistical notation. Looking forward for your answers, thank you in advance.

 A: It looks like "kurtosis" to me.
Statisticians use a few special numbers to describe the distribution of a data set. Most people aware of the "mean" - it tells you where the data is centred. The "variance" is a measure of the spread of the data - how close is it to the mean? The "skewness" tells you if the data is spread evenly on both sides of the mean or if in fact there is a tendency to be on one side or the other. The "kurtosis" tells you if the central peak of the data is relatively high or low compared to the tails.
Together these are referred to as the moments of the data, with clear links to moments and moments of inertia as used in mechanics.
A good clue that a formula is linked to the higher moments is the presence of terms involving $x_i - \bar x$.
For a normal distribution, the mean is 0, the variance (and therefore standard deviation) is 1, the skewness is 0 (although I wish it was 2) and the kurtosis is 3.
Many people want to compare their data set with the normal distribution, so they calculate what is properly called the excess kurtosis, which is kurtosis - 3. However, this is often referred to as kurtosis!
This is a good place to find out more. 
