A pulse train is a very useful signal, so it's a good idea to understand the Fourier series coefficients for this signal.
In this case, we define the pulse to have a width of T1 (several books use a width of 2T1, but I find that T1 is a little more revealing).
In essence the Fourier series coefficients are the values of a sinc function.
This function will come in very useful in several situations, so it's a good idea to get to know it.
