Mathematical formulation of periodogram built-in function

I used periodogram(data, hamming, [], freq) to generate autospectrum of a data. Where do I find the mathematical model of the above function - I would like to know the mathematical expressions used in the above calculations to write my research work.

The sample of my code is here where turbw is the data.

[pww, fww] = periodogram(turbw, hamming(length(turbw)), [],freq);

pww = smooth(pww);

sm1 = floor(length(fww)/20);

pww(1:sm1) = smooth(pww(1:sm1));

pww(sm1:end) = smooth(pww(sm1:end));

Answer 1

[Sai Veeramachaneni]

You can find the required information here.