Maths for ML / Notation

σ, σ2, σ(z)\sigma,\ \sigma^2,\ \sigma(z)

sigma

Greek letters and conventionsBeginner

Say it “sigma / sigma squared / sigmoid of z”

Lowercase σ is the standard deviation (σ² the variance) — or, as a function σ(z), the sigmoid. Capital Σ is a sum or a covariance matrix.

Example

σ(z)=11+e−z\sigma(z) = \frac{1}{1+e^{-z}}

The sigmoid function.

In code

x.std()                # σ, the standard deviation
1 / (1 + np.exp(-z))   # σ(z), the sigmoid
x.std()           # σ, the standard deviation
torch.sigmoid(z)  # σ(z), the sigmoid

Where you’ll see it in ML

Normalization layers, logistic regression, gates.

Don’t confuse it with

Lessons that use it

← All 94 symbols