Sigma notation is a mathematical notation to write long sums in a short way. Sigma notation uses the Greek letter Sigma (formula_3), and takes upper and lower bounds which tell us where the sum begins and where it ends. The lower bound usually has a variable (called the index, often denoted by formula_4, formula_5 or formula_6) along with a value, such as "formula_7". This tells us that the summation begins at 2, and goes up by 1 until it reaches the number on the top.