"in" mathematically represents the base of natural logarithm, that is, e. E is an irrational number, which is approximately equal to 7 1828. Its definition can be regarded as a special form of limit, that is, e can be regarded as limit lim (1+ 1/n) n, where n approaches infinity. Because e is the base of natural logarithm, special operations of natural logarithm, such as derivation, are closely related to e.

The base e of natural logarithm is an extremely important number in mathematics, and it is widely used, especially in the fields of probability theory, calculus, compound interest and so on. The practical application of E can range from biology, physics and chemistry to economics and computer science, which greatly facilitates the calculation and modeling of practical problems by scientists and engineers. More application fields and principles need further in-depth study and research.

In addition to the base of natural logarithm, in also has other meaning in mathematics. For example, in complex numbers, it represents that the imaginary unit I. I is a very special number, $ i 2 =-1$,which is different from all other numbers in the real number set. The introduction of imaginary number fills the defect that real number can't calculate the square root of negative number, and expands the research in the field of mathematics.