I am struggling with the basic theory behind summations and products of the series of numbers can anybody give me a link to a site with some theory or give me some general rules.

Example

log(product series from i = 1 to n of ((lambda ^xi)/xi!)

= summation from i = 1 to n of (xi * log lambda - log (xi!))

how do you get from the product of the numbers to the summation?????