Totative
A Positive Integer less than or equal to a number 
which is also Relatively Prime to , where 1is counted as being Relatively Prime to all numbers. The number of totatives of is the value of theTotient Function 
.
- Rudin-Shapiro Sequence
- Rudvalis Group
- Rule
- Ruled Surface
- Rule of 72
- Ruler
- Ruler Function
- Ruling
- Run
- Runge-Kutta Method
- Runge-Walsh Theorem
- Running Knot
- Russell's Antinomy
- Russell's Paradox
- Russian Multiplication
- Russian Roulette
- Ryser Formula
- Rényi's Parking Constants
- Rössler Model
- Saalschützian
- Saalschütz's Theorem
- s-Additive Sequence
- Saddle
- Saddle-Node Bifurcation
- Saddle Point (Fixed Point)