What it means
The eighteenth Greek letter. Lower-case σ is the standard deviation (σ² is the variance), stress in materials and the sigmoid function. Capital Σ is a sum of many terms, and also a covariance matrix.
Read aloud in real formulas
σ² = (1/n) ∑ᵢ (xᵢ − μ)²
Aethel says
sigma squared equals the quantity 1 over n, times the sum over i of the quantity x sub i minus mu, squared
the variance
σ(x) = 1 / (1 + e^(−x))
Aethel says
sigma of x equals 1 over the quantity 1 plus e to the power of negative x
the sigmoid function
∑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
capital sigma as a sum
Typing it
- LaTeX
- \sigma, \Sigma
- Unicode
- U+03C3, U+03A3
Formulas that use it
Questions
How do you pronounce sigma (σ)?
It is said “sigma”, roughly SIG-muh.
What does σ mean in maths?
The eighteenth Greek letter. Lower-case σ is the standard deviation (σ² is the variance), stress in materials and the sigmoid function. Capital Σ is a sum of many terms, and also a covariance matrix.
How do you type σ?
In LaTeX, write \sigma, \Sigma. Its Unicode code point is U+03C3, U+03A3.