The formula means
From n 1 to ni.
A symbol commonly used in mathematics, mainly used to find the sum of polynomials, expressed by ∑.
The small characters below ∑, such as i= 1, indicate that the sum starts from i= 1.
The little words above, such as n, mean add up to n.
For example, i= 1 is written below, N is written above, and Xi is written behind (I is the lower corner mark).
It means starting from x 1+x2+ ... +xn.
For example, i=5 is written below, N is written above, and Xi is written behind (I is the lower corner mark).
It means from x5+x6+…+xn.