Inclusion-Exclusion Principle
If 
, ..., 
are finite sets, then
where
is the sum of the Cardinalities of the intersections of the sets taken 
at a time.- Argument Addition Relation
- Argument (Complex Number)
- Argument (Elliptic Integral)
- Argument (Function)
- Argument Multiplication Relation
- Argument Principle
- Argument Variation
- Aristotle's Wheel Paradox
- Arithmetic
- Arithmetic-Geometric Mean
- Arithmetic Geometry
- Arithmetic-Harmonic Mean
- Arithmetic-Logarithmic-Geometric Mean Inequality
- Arithmetic Mean
- Arithmetic Progression
- Arithmetic Sequence
- Arithmetic Series
- Armstrong Number
- Arnold Diffusion
- Arnold's Cat Map
- Arnold Tongue
- Aronhold Process
- Aronson's Sequence
- Arrangement
- Arrangement Number