Compare the possible meanings
| Notation | Typical meaning | How to identify it |
|---|---|---|
x ∈ A | Membership | Left side is an object |
B ⊆ A | Subset | Both sides are sets |
x ∉ A | Not an element | Membership denied |
B ⊈ A | Not a subset | Containment denied |
Examples
- 2∈{1,2,3}.
- {1,2}⊆{1,2,3}.
- Writing 2⊂A is normally a type error unless 2 denotes a set.
How to decide
Identify the type of the left operand: an element calls for ∈; a set calls for a subset relation.
How to verify the interpretation
Rewrite the statement in words and confirm the left and right objects have compatible types.