Epsilons, no. 6: What is that Σ?

The capital Greek sigma, that is, repeated addition

One of the most common questions I get: “What is that Σ symbol?“

After 16 years of working with them, I routinely use the “Σ symbol”, a.k.a. the sum, but it was not always like that. As a freshman, I recall watching in terror the multitude of sums stacked on each other, feeling utterly confused about what was in front of me. Being comfortable with summation is not the default.

So, I am painfully aware of what you are feeling.

To help you, here’s a brief guide, with an infographic at the end. (If you are already comfortable with summation, you might want to skip this issue.)

By definition, the capital Greek sigma (Σ) symbolizes repeated addition, which is called summation. We mainly use this to sum sequences, that is, numbers indexed by integers.

In most programming languages, summation can be implemented via a for loop. (There are other approaches, like in…