What is that weird symbol (boxed in pink in the screenshot below) called? I'd like to know the name so I can learn more about it on YouTube. Thanks!
Surely you know that $\sum\limits_{k = 1}^9 k =45$ represents the sum of the integers one to nine.
The symbol $\sum$ is the capital Greek letter sigma for sum.
Well $\prod\limits_{k = 1}^9 k = 362880$ is the product of the integers one to nine.
The symbol $\Pi$ is the Greek letter capital pi for product.