Gregory Number
A number
where
is an Integer or Rational Number, 
is the Inverse Tangent, and 
is the Inverse Cotangent. Gregory numbers arise in the determination of Machin-Like Formulas.Every Gregory number 
can be expressed uniquely as a sum of 
s where the 
s are Stérmer Numbers.
References
Conway, J. H. and Guy, R. K. ``Gregory's Numbers'' In The Book of Numbers. New York: Springer-Verlag, pp. 241-242, 1996.
- Span (Link)
- Span (Polynomial)
- Span (Set)
- Span (Vectors)
- Sparse Matrix
- Spearman Rank Correlation Coefficient
- Special Curve
- Special Function
- Special Linear Group
- Special Matrix
- Special Orthogonal Group
- Special Point
- Special Series Theorem
- Special Unitary Group
- Species
- Specificity
- Spectral Norm
- Spectral Power Density
- Spectral Radius
- Spectral Rigidity
- Spectral Theorem
- Spectrum (Operator)
- Spectrum Sequence
- Speed
- Spencer's 15-Point Moving Average