The two vertical lines indicate that the mathematical meaning of this formula is norm, the 2 of the base angle indicates 2 norm, and the 2 above indicates the square of 2 norm.

According to the type of formula, it can be divided into the following two specifications:

Matrix 2 norm: the 2 norm of matrix A is the maximum value of the transposed root number of a multiplied by the characteristic root of matrix A;

Vector 2 norm: the 2 norm of vector x is the sum of squares of all elements in x and then open the root sign;

Function 2 norm: The 2 norm of function f(x) is the root sign of the integral of the square of f(x) in the interval (a, b).