Lyapunov's First Theorem
A Necessary and Sufficient condition for all the Eigenvalues of a Real 
matrix A to have Negative Real Parts is that the equation
has as a solution where
is an matrix and 
is a positive definite quadraticform.
References
Gradshteyn, I. S. and Ryzhik, I. M. Tables of Integrals, Series, and Products, 5th ed. San Diego, CA: Academic Press, p. 1122, 1979.
- Routh-Hurwitz Theorem
- Routh's Theorem
- RSA Encryption
- RSA Number
- Rubber-Sheet Geometry
- Rubik's Clock
- Rubik's Cube
- 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