Interval Notation Bracket Rules
When to use parentheses vs square brackets:
OPEN ( , )
Parenthesis
Used for strict inequalities (<, >) where the endpoint is EXCLUDED.
CLOSED [ , ]
Square Bracket
Used for non-strict inequalities (≤, ≥) where endpoint is INCLUDED.
INFINITY
Always Open
Positive and negative infinity (-∞, ∞) ALWAYS take parentheses.
HALF-OPEN
Mixed Brackets
Combinations like [a, b) or (a, b] for asymmetric bounds.