The Greek letter sigma is the commonly used mathematical symbol for population standard deviation. The Greek letter mu is also commonly used in calculations of the population standard deviation for non-overlapping populations.
The formula for calculating the population standard deviation is the sum of all values from i to N, where i is the initial value and N is the last value, as evaluated by the function of the square of the initial x value minus the value of mu. That sum is multiplied by 1/N, with the square root of the product providing the value for the population standard deviation represented by sigma. Mu is the mean of all values being considered for x.