Sigma-ideal

In the context of set theory and measure theory, a **σ-ideal** (sigma-ideal) is a specific type of collection of sets that satisfies certain properties concerning the operations of countable unions and subsets. More formally, a family \( I \) of subsets of a set \( X \) is called a σ-ideal if it satisfies the following conditions: 1. **Non-empty:** The empty set is an element of \( I \), i.e.