I think I understand the basic concept of sampling theorem, that is that the sampling frequency must at least twice the highest frequency of the function being sampled. I have been watching DSP lectures on youtube and in the video the lecturer presents a sampling theorem example that has confused me. The question is: 3 cosine waves of different frequencies are to be sampled. cos(6πn), cos(14πn) and cos(26πn). All 3 functions are to be sampled at a frequency of 10 Hz. This make T = 0.1s. Multiplying omega (ω= 2πf) by the sampling rate the three functions become. cos(0.6πn), cos(1.4πn) and cos(2.6πn). Only the function with 3Hz original frequency is sampled at the correct rate, the second and third functions are under sampled, therefore aliasing will occur. However the lecturer stated that: "1.4πn is 2πn - 1.4πn which equals 0.6πn and 2.6πn is 2πn + 0.6πn which also equals 0.6πn" All cosine waves will appear to have the same frequency. I don't understand how he is doing the addition and subtraction to obtain the aliased frequency (speech marks bit). If anyone can give me a hint or clue I will be very great full. example starts at 37.10 minutes. http://www.youtube.com/watch?v=JpHXMcDxNiA&feature=relmfu Thanks.