As a mathematical constant, e is the basis of natural logarithmic function. Sometimes called Euler number, named after the Swiss mathematician Euler; There is also a relatively rare name Napier constant to commemorate the introduction of logarithm by Scottish mathematician John Napier. Its value is about (20 decimal places): e ≈ 2.7182818284 59045 23536 ... Just like pi and imaginary unit I, e is one of the most important constants in mathematics. It has several equivalent definitions, some of which are listed below: directory [hidden] 1 definition 2 nature 3 irrational number proof 4 E's history outside mathematics uses 6 known digits 7. See [edit] definition. The four most common definitions of e are as follows: 1. E is defined as the following limit:. & lt/DD & gt； 2. define e as the sum of the following infinite series: 0 makes. & lt/DD & gt； 4. Define E as the unique number generated by X.