Bankoff Circle
and 
in the Arbelos figure, there is a third circle 
congruent to these two as illustrated in the above figure.See also Arbelos
References
Bankoff, L. ``Are the Twin Circles of Archimedes Really Twins?'' Math. Mag. 47, 214-218, 1974. Gardner, M. ``Mathematical Games: The Diverse Pleasures of Circles that Are Tangent to One Another.'' Sci. Amer. 240, 18-28, Jan. 1979.
- Least Squares Fitting--Exponential
- Least Squares Fitting--Logarithmic
- Least Squares Fitting--Power Law
- Least Upper Bound
- Lebesgue Constants (Fourier Series)
- Lebesgue Constants (Lagrange Interpolation)
- Lebesgue Covering Dimension
- Lebesgue Dimension
- Lebesgue Integrable
- Lebesgue Integral
- Lebesgue Measurability Problem
- Lebesgue Measure
- Lebesgue Minimal Problem
- Lebesgue-Radon Integral
- Lebesgue Singular Integrals
- Lebesgue-Stieltjes Integral
- Lebesgue Sum
- Le Cam's Identity
- Leech Lattice
- Lefshetz Fixed Point Formula
- Lefshetz's Theorem
- Lefshetz Trace Formula
- Leg
- Legendre Addition Theorem
- Legendre Differential Equation