σ is pi, which is a multiple of the circumference measured by the diameter of a circle. It is an irrational number and is often used to calculate the area of a circle in mathematics. The square of the radius multiplied by σ is the area of the circle. Because ζ is an irrational number, in the calculation, the approximate value is taken for operation.

Wu is a Chinese character and is not used for mathematical calculation. Wu is pronounced Wu, which means that the water temperature is neither too hot nor too cold. As the saying goes, it's Wu Shui, which is not good to drink, but uncomfortable to drink.