- Abel's test - see Series - Theory - Testing convergence - Convergence of general series
- Abel's convergence theorem - see Series - Theory - Series of functions - Power series
- Abel's uniform convergence test - see Series - Theory - Series of functions - Series of functions
- absolute convergence of series - see Series - Theory - Introduction - Absolute convergence
- absolute value
- accumulation point - see Functions - Theory - Real numbers - Topological notions
- Achilles and tortoise - see this note
- add-subtract trick - see Extra - Long division and the add-subtract trick
- adding functions - see Functions - Theory - Real functions - Operations
- adding power series - see Series - Theory - Series of functions - Power series
- adding sequences - see Sequences - Theory - Introduction - Operations, and limit: Sequences - Theory - Limit - Properties
- adding series of real numbers - see Series - Theory - Introduction - Basic properties
- algebra of DNE (functions) - see this note
- algebra of DNE (sequences) - see this note
- algebra of limits (functions) - see Functions - Theory - Limit - Basic Properties, also this note
- algebra of limits (sequences) - see Sequences - Theory - Limit - Operations, also this note
- alternating sequence - definition in Sequences - Theory - Introduction - Important examples, properties in Sequences - Theory - Limit - Important examples
- alternating series - see Series - Theory - Introduction - Important examples
- alternating series, approximating sum - see Series - Theory - Introduction - Approximating sums
- Alternating series test - see Series - Theory - Testing convergence - Convergence of general series
- amplitude-phase form of Fourier series - see Series - Theory - Series of functions - Properties of Fourier series
- angles in plane - see this note
- antiderivative - see Integrals - Theory - Introduction - Newton integral
- antisymmetry - see Extra - Sets and mappings - Important sets of numbers
- approximation of functions - see Derivatives - Theory - Applications - Approximation of functions by tangent lines and Derivatives - Theory - Applications - Taylor polynomial, also Derivatives - Methods Survey - Applications - Taylor polynomial and approximation
- approximating series - see Series - Theory - Introduction - Approximating sums and Series - Methods Survey - Summing up series - Approximating series
- Archimedes spiral - see Functions - Theory - Implicit and parametric functions - Parametric functions
- arcsin/arccos/arctan/arccot
- area
- argsinh/argcosh/argtanh/argcoth
- argument of a function - see Functions - Theory - Real functions - Introduction
- arithmetic sequence - definition in Sequences - Theory - Introduction - Important examples, properties in Sequences - Theory - Limit - Important examples
- associative law - see Extra - Sets and mappings - Important sets of numbers
- associative law for series - see Series - Theory - Introduction - Basic properties
- AST - see Alternating series test in Series - Theory - Testing convergence - Convergence of general series
- asymptotes (general) - see Functions - Theory - Limit - Order of functions, asymptotes
- asymptotes (horizontal, vertical, oblique) - see Derivatives - Theory - Graphing functions - Asymptotes and Derivatives - Methods Survey - Graphing functions - Asymptotes
- asymptotic function - see Functions - Theory - Limit - Order of functions, asymptotes
- average of a function - see Integrals - Theory - Applications - Average