>> Another notation used in the study of sequences is summation notation. Now, summation notation is also referred to often a sigma notation because it uses the upper case Greek letter, sigma. So, sigma meaning sum now in the mathematical sense. Now here's how sigma notation works. We have this Greek letter of sigma and at the bottom we have a letter set equal to the number. And that letter appears in an expression over here. Now, we also have a number at the top of the Greek Letter sigma. This is called the index of the summation. This is the lower limit and that's the upper limit of the summation, and we're just adding a bunch of things. And the things that were added are generated by using this expression and replacing K with all of the values from this lower limit to the upper limit, you see. So this means we're adding this expression when K is replaced with 1, and plus this expression when K is replaced with 2, plus this expression when K is replaced with the upper limit 3. You see this is a rather narrow group of things being added but nonetheless, that's the idea. So to expand this, we would take this expression and replace the letter in it with the lower limit value and then replace that index with every value up to the upper limit, so it would be 2 times 1 plus 1, plus 2 times 2 plus1, plus two times 3 plus 1, and then we would evaluate to find that to be 15.