Summation notation is used to compactly represent a sum of numbers. For example, suppose we want to compactly write the following sum, 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15. Sums of numbers, such as the one above, are often called series.