A Spectrum formed by the Lagrange Numbers. The only ones less than three are the Lagrange Numbers, but the last gaps end at Freiman's Constant. Real Numbers larger than Freiman's Constant are in the Markov Spectrum.
See also Freiman's Constant, Lagrange Number (Rational Approximation), Markov Spectrum, Spectrum Sequence
References
Conway, J. H. and Guy, R. K. The Book of Numbers. New York: Springer-Verlag, pp. 187-189, 1996.