Sum
Sigma notation is a mathematical notation to write long sums in a short way. Sigma notation uses the Greek letter Sigma (), 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 ,  or ) along with a value, such as "". This tells us that the summation begins at 2, and goes up by 1 until it reaches the number on the top.