# 4.05: Important Constants

- Page ID
- 1104

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##### Euler’s number

Euler’s constant is an important value for exponential functions, especially scientific applications involving decay (such as the decay of a radioactive substance). It is especially important in calculus due to its uniquely self-similar properties of integration and differentiation.

##### Pi

Pi (π) is defined as the ratio of a circle’s circumference to its diameter.

** Note:** For both Euler’s constant (

*e*) and pi (π), the spaces shown between each set of five digits have no mathematical significance. They are placed there just to make it easier for your eyes to “piece” the number into five-digit groups when manually copying.