Symbols · Operators and calculus
∑ Summation (sigma notation)
Say it: “the sum from i equals 1 to n of”, “sum over i”, “sigma”
What it means
A capital sigma means: add up the expression after it for every value of the index. The limits below and above give the first and last values; a subscript alone, such as ∑ᵢ, means over every i.
Read aloud in real formulas
∑ᵢ₌₁ⁿ i = n(n + 1)/2
Aethel says
the sum from i equals 1 to n of i equals n times the quantity n plus 1, over 2
the sum of the first n whole numbers
x̄ = (1/n) ∑ᵢ xᵢ
Aethel says
x bar equals the quantity 1 over n, times the sum over i of x sub i
the mean of n values
Typing it
- LaTeX
- \sum_{i=1}^{n}
- Unicode
- U+2211
Formulas that use it
Questions
How do you say ∑ out loud?
It is usually read “the sum from i equals 1 to n of”, and also “sum over i” or “sigma”.
What does ∑ mean in maths?
A capital sigma means: add up the expression after it for every value of the index. The limits below and above give the first and last values; a subscript alone, such as ∑ᵢ, means over every i.
How do you type ∑?
In LaTeX, write \sum_{i=1}^{n}. Its Unicode code point is U+2211.
What is ∑ often confused with?
It is easy to mistake for the Greek letter Σ (U+03A3), which looks the same and is used for covariance matrices.