Summation notation and sizing the kanban bus route
Σ or Sigma is the Greek upper case capital letter S and is used in mathematics to represent summation or addition of a series of elements or set of data values.
x1, x2, x3, ... xn is a set of numbers. x1 is the first number in the set, xi is the 'i'th number, xn is the last of n numbers.
Elements can be simply adding up a set, or more involved such as summing squares or other algebraic equations.