∑ is a summation function, pronounced sigma. A general symbol has a number above it, such as Y, and a formula below it, such as n=x, where X and Y are specific numbers, and N is the variable in the following expression, which together represent the range of N, followed by an expression including the variable N. The whole combination means: something given up and down. Find the results of the expression after the symbol according to different n, and then sum these results. Sometimes, you just need to write a formula similar to the following n=[x, y] to represent the range of variables.