Sum
The result of adding a list of numbers, written compactly with sigma.
- Linear: Σ c·a_i = c·Σ a_i.
- Σ (a_i + b_i) = Σ a_i + Σ b_i.
- Most common notation in ML papers.
When to use: Aggregating losses, gradients, counts across a batch or dataset.
Example: Batch total: Σ_{i=1}^{32} L(y_i, ŷ_i). Average: divide by 32.