Gauss's Harmonic Function Theorem
If a function 
is Harmonic in a Sphere, then the value of at the centerof the Sphere is the Arithmetic Mean of its value on the surface.
- Arrangement
- Arrangement Number
- Array
- Arrowhead Curve
- Arrow Notation
- Arrow's Paradox
- Art Gallery Theorem
- Articulation Vertex
- Artin Braid Group
- Artinian Group
- Artinian Ring
- Artin L-Function
- Artin Reciprocity
- Artin's Conjecture
- Artin's Constant
- Artin's Reciprocity Theorem
- Artistic Series
- ASA Theorem
- Aschbacher's Component Theorem
- A-Sequence
- Associative
- Associative Algebra
- Associative Magic Square
- ASS Theorem
- Astroid