The capital Greek letter Pi is used to represent the product operator in mathematics. The operator has three parts: an initial value, an end value, and the expression being evaluated. To evaluate the result, the expression starts at the initial state and increments the counting variable until the end value is reached. Each subsequent expression is multiplied together to get the final result.
For example, the expression below starts at the variable is equal to one and iterates until is equal to three. At each iteration the expression is evaluated with the current value of and multiplied with the prior expressions.